diff --git a/src/App.vue b/src/App.vue
index f060725..32e08c5 100644
--- a/src/App.vue
+++ b/src/App.vue
@@ -4,7 +4,7 @@ import TitleBar from "./components/titlebar-element.vue";
 import BottomNavbar from "./components/bottom-navbar.vue";
 import DateSelector from "./components/date-selector.vue";
 import LoadingElement from "./components/loading-element.vue";
-import { loading, loadingProgress, theme } from "./store";
+import { loading, loadingProgress, loadingFailed, theme } from "./store";
 import { computed, ref } from "vue";
 
 const autoThemes = { true: "dark", false: "light" };
@@ -31,7 +31,11 @@ const isDataView = computed(() => {
     :class="theme == 'auto' ? `theme-${autoTheme}` : `theme-${theme}`"
   >
     <TitleBar />
-    <LoadingElement :active="loading" :progress="loadingProgress" />
+    <LoadingElement
+      :active="loading"
+      :progress="loadingProgress"
+      :error="loadingFailed"
+    />
     <div class="center">
       <main>
         <DateSelector v-show="isDataView" />
diff --git a/src/components/history-list.vue b/src/components/history-list.vue
index 4fc074d..b97f0d7 100644
--- a/src/components/history-list.vue
+++ b/src/components/history-list.vue
@@ -1,5 +1,5 @@
 <script setup>
-import { historyOfDate } from "../store";
+import { history } from "../store";
 import { getSubstitutionText } from "../util";
 import { computed } from "vue";
 import dayjs from "dayjs";
@@ -10,39 +10,27 @@ const props = defineProps({
   },
 });
 
-const history = computed(() => {
-  return historyOfDate.value[props.date.setUTCHours(0, 0, 0, 0)];
+const historyOfDate = computed(() => {
+  return history.value[props.date.getTime()];
 });
 
-function getChar(type) {
-  switch (type) {
-    case "change":
-      return "~";
-    case "addition":
-      return "+";
-    case "deletion":
-      return "-";
-  }
-}
-
-function getColor(type) {
-  switch (type) {
-    case "change":
-      return "background-color: var(--substitution-background-change)";
-    case "addition":
-      return "background-color: var(--substitution-background-addition);";
-    case "deletion":
-      return "background-color: var(--substitution-background-deletion);";
-  }
-}
+const chars = {
+  change: "~",
+  addition: "+",
+  deletion: "-",
+};
 </script>
 
 <template>
   <div class="history">
-    <template v-for="event in history" :key="event">
-      <div class="change" :style="getColor(event.type)">
-        <span class="hour">{{ event.lesson }}{{ getChar(event.type) }}</span>
+    <template v-for="event in historyOfDate" :key="event">
+      <div
+        class="change"
+        :style="`background-color: var(--substitution-background-${event.type}`"
+      >
+        <span class="hour">{{ event.lesson }}{{ chars[event.type] }}</span>
         <div class="infos">
+          <!-- If the entry is a change show which values changed -->
           <span class="text" v-if="event.type == 'change'">
             <template v-for="(change, key) in event.change" :key="key">
               <p>
@@ -52,6 +40,7 @@ function getColor(type) {
               </p>
             </template>
           </span>
+          <!-- If the entry is an addition or deletion generate a text -->
           <span class="text" v-else
             >{{
               $t(getSubstitutionText(event.change), {
diff --git a/src/components/substitution-list.vue b/src/components/substitution-list.vue
index 515a5e7..21b1e97 100644
--- a/src/components/substitution-list.vue
+++ b/src/components/substitution-list.vue
@@ -1,5 +1,5 @@
 <script setup>
-import { substitutionsForDate } from "../store";
+import { substitutions } from "../store";
 import { getSubstitutionText, getSubstitutionColor } from "../util";
 import { computed } from "vue";
 
@@ -9,13 +9,13 @@ const props = defineProps({
   },
 });
 
-const substitutions = computed(() => {
-  return substitutionsForDate.value[props.date.setUTCHours(0, 0, 0, 0)];
+const substitutionsForDate = computed(() => {
+  return substitutions.value[props.date.getTime()];
 });
 </script>
 
 <template>
-  <template v-for="substitution in substitutions" :key="substitution">
+  <template v-for="substitution in substitutionsForDate" :key="substitution">
     <div class="substitution" :style="getSubstitutionColor(substitution)">
       <span class="hour">{{ substitution.lesson }}</span>
       <div class="infos">
diff --git a/src/components/timetable-list.vue b/src/components/timetable-list.vue
index 629529f..caa2344 100644
--- a/src/components/timetable-list.vue
+++ b/src/components/timetable-list.vue
@@ -17,14 +17,17 @@ const linkedTimetable = computed(() => {
   const newDay = currentDay.map((e, index) => {
     const newElement = { ...e };
     // Find a substitution mathing this lesson
-    newElement.substitution = substitutions.value.find((entry) => {
-      const entryDay = new Date(entry.date).getTime();
-      return (
-        entry.lesson == index + 1 &&
-        entryDay == props.date.getTime() &&
-        (entry.teacher == e.teacher || !entry.teacher || !e.teacher)
+    if (substitutions.value[props.date.getTime()])
+      newElement.substitution = substitutions.value[props.date.getTime()].find(
+        (entry) => {
+          const entryDay = new Date(entry.date).getTime();
+          return (
+            entry.lesson == index + 1 &&
+            entryDay == props.date.getTime() &&
+            (entry.teacher == e.teacher || !entry.teacher || !e.teacher)
+          );
+        }
       );
-    });
     return newElement;
   });
   return newDay;
diff --git a/src/store.js b/src/store.js
index 901bbd9..ffd11d4 100644
--- a/src/store.js
+++ b/src/store.js
@@ -1,11 +1,15 @@
 import { ref, watch, computed } from "vue";
 import router from "./router";
 import i18n from "./main";
+import { getNextAndPrevDay } from "./util";
 
+/* Router */
 export const lastRoute = ref();
 export const loading = ref(false);
 export const loadingProgress = ref(0);
+export const loadingFailed = ref(false);
 
+/* Preferences */
 export const classFilter = ref(localStorage.getItem("classFilter") || "none");
 export const timetableGroups = ref(
   JSON.parse(localStorage.getItem("timetableGroups") || "[]")
@@ -29,12 +33,7 @@ watch(theme, (newValue) => {
   localStorage.setItem("theme", newValue);
 });
 
-// Set this to a positive or negative integer
-// to change selectedDate by this number
-export const changeDay = ref(0);
-// Set this to jump to a specific date
-export const changeDate = ref(new Date());
-
+/* Date selector */
 export const selectedDate = ref(new Date(new Date().setUTCHours(0, 0, 0, 0)));
 export const selectedDay = computed(() => selectedDate.value.getDay() - 1);
 // Jump to next Monday if it is weekend
@@ -43,109 +42,73 @@ if (selectedDay.value == 5)
 if (selectedDay.value == -1)
   selectedDate.value = new Date(selectedDate.value.getTime() + 86400000);
 
-// Load new data if date changes
-watch(selectedDate, async () => {
-  loadingProgress.value = 0.1;
-  loading.value = true;
-  await fetchSubstitutions();
-  loadingProgress.value = 1 / 2;
-  await fetchHistory();
-  loadingProgress.value = 1;
-  loading.value = false;
-});
+// Set this to a positive or negative integer
+// to change selectedDate by this number
+export const changeDay = ref(0);
+// Set this to jump to a specific date
+export const changeDate = ref(new Date());
 
+/* Data store */
 export const timetable = ref({ trusted: true });
-export const substitutions = ref([]);
-export const history = ref([]);
-export const classList = ref([]);
-
-export const historyOfDate = computed(() => {
-  const dates = {};
-  for (const entry of history.value) {
-    const date = entry.date;
-    if (!dates[date]) dates[date] = [];
-    dates[entry.date].push(entry);
-  }
-  return dates;
-});
-
-export const substitutionsForDate = computed(() => {
-  const dates = {};
-  for (const substitution of substitutions.value) {
-    const date = substitution.date;
-    if (!dates[date]) dates[date] = [];
-    dates[substitution.date].push(substitution);
-  }
-  return dates;
-});
-
-export const parsedTimetable = computed(() => {
-  if (!timetable.value.data) return [];
-  return timetable.value.data.map((day) => {
-    const newDay = [];
-    for (const lesson of day) {
-      var usedLesson = lesson;
-      // Check for timetable groups
-      if (Array.isArray(lesson)) {
-        var matchingLesson = lesson.find((e) =>
-          timetableGroups.value.includes(e.group)
-        );
-        if (!matchingLesson) {
-          matchingLesson = {
-            subject: lesson.map((e) => e.subject).join(" / "),
-            teacher: i18n.global.t("timetable.configureTimetableGroup"),
-            length: lesson[0].length || 1,
-          };
-        }
-        usedLesson = matchingLesson;
-      }
-      const lessonLength = usedLesson.length || 1;
-      delete usedLesson.length;
-      for (var i = 0; i < lessonLength; i++) newDay.push(usedLesson);
-    }
-    return newDay;
-  });
-});
-
-export const possibleTimetableGroups = computed(() => {
-  const foundTimetableGroups = [];
-  if (!timetable.value.data) return [];
-  for (const day of timetable.value.data) {
-    for (const lesson of day) {
-      if (Array.isArray(lesson)) {
-        for (const group of lesson) {
-          if (!foundTimetableGroups.includes(group.group)) {
-            foundTimetableGroups.push(group.group);
-          }
-        }
-      }
-    }
-  }
-  return foundTimetableGroups;
-});
+export const substitutions = ref({});
+export const history = ref({});
+export const classList = ref({});
 
+/* API functions */
+// Set the `VITE_API_ENDPOINT` env variable when
+// building the frontend to use an external api server
 const baseUrl = import.meta.env.VITE_API_ENDPOINT || "/api";
 
-export async function fetchData() {
+export async function fetchData(days, partial) {
+  const steps = 2 * days.length + (partial ? 0 : 2);
+  let step = 1;
+  loadingFailed.value = false;
   loadingProgress.value = 0.1;
   loading.value = true;
 
-  const checkResponse = await fetch(`${baseUrl}/check`);
-  if (checkResponse.status != 200) router.push("/login");
-  loadingProgress.value = 1 / 5;
+  // Check if the API server is reachable
+  // and the user is authenticated
+  try {
+    const checkResponse = await fetch(`${baseUrl}/check`);
+    if (checkResponse.status == 401) {
+      router.push("/login");
+      return;
+    } else if (checkResponse.status != 200) {
+      loadingFailed.value = true;
+      loadingProgress.value = 1;
+      console.log("Other error while fetching data: " + checkResponse.status);
+      return;
+    }
+  } catch {
+    loadingFailed.value = true;
+    loadingProgress.value = 1;
+    console.log("Error while fetching data: No internet connection!");
+    return;
+  }
+  loadingProgress.value = step++ / steps;
 
-  await fetchClassList();
-  loadingProgress.value = 2 / 5;
-  await fetchTimetable();
-  loadingProgress.value = 3 / 5;
-  await fetchSubstitutions();
-  loadingProgress.value = 4 / 5;
-  await fetchHistory();
+  if (!partial) {
+    await fetchClassList();
+    loadingProgress.value = step++ / steps;
+    await fetchTimetable();
+    loadingProgress.value = step++ / steps;
+  }
+  for (const day of days) {
+    await fetchSubstitutions(day);
+    loadingProgress.value = step++ / steps;
+    await fetchHistory(day);
+    loadingProgress.value = step++ / steps;
+  }
 
   loadingProgress.value = 1;
   loading.value = false;
 }
 
+// Load new data if date changes
+watch(selectedDate, () =>
+  fetchData(getNextAndPrevDay(selectedDate.value), true)
+);
+
 export async function fetchClassList() {
   const classListResponse = await fetch(`${baseUrl}/classes`);
   const classListData = await classListResponse.json();
@@ -163,19 +126,19 @@ export async function fetchTimetable() {
   } else timetable.value = timetableData;
 }
 
-export async function fetchSubstitutions() {
-  const requestDate = `?date=${selectedDate.value.getTime()}`;
+export async function fetchSubstitutions(day) {
+  const requestDate = `?date=${day}`;
   const substitutionResponse = await fetch(
     classFilter.value == "none"
       ? `${baseUrl}/substitutions${requestDate}`
       : `${baseUrl}/substitutions${requestDate}&class=${classFilter.value}`
   );
   const substitutionData = await substitutionResponse.json();
-  substitutions.value = substitutionData;
+  substitutions.value[day] = substitutionData;
 }
 
-export async function fetchHistory() {
-  const requestDate = `?date=${selectedDate.value.getTime()}`;
+export async function fetchHistory(day) {
+  const requestDate = `?date=${day}`;
   const historyResponse = await fetch(
     classFilter.value == "none"
       ? `${baseUrl}/history${requestDate}`
@@ -183,7 +146,63 @@ export async function fetchHistory() {
   );
   const historyData = await historyResponse.json();
   if (historyData.error) console.warn("API Error: " + historyData.error);
-  else history.value = historyData;
+  else history.value[day] = historyData;
 }
 
-fetchData();
+/* Preprocess the timetable data */
+export const parsedTimetable = computed(() => {
+  // Check if timetable data exists
+  if (!timetable.value.data) return [];
+  return timetable.value.data.map((day) => {
+    const parsedDay = [];
+    for (const lesson of day) {
+      let usedLesson = lesson;
+      // Check if lesson has multiple options
+      // (timetable groups)
+      if (Array.isArray(lesson)) {
+        let matchingLesson = lesson.find((e) =>
+          timetableGroups.value.includes(e.group)
+        );
+        // If no valid timetable group is configured
+        // add a dummy lesson showing a notice
+        if (!matchingLesson) {
+          matchingLesson = {
+            subject: lesson.map((e) => e.subject).join(" / "),
+            teacher: i18n.global.t("timetable.configureTimetableGroup"),
+            length: lesson[0].length || 1,
+          };
+        }
+        usedLesson = matchingLesson;
+      }
+      // Duplicate the lesson if its length is > 1 for it
+      // to show up multiple times in the timetable view
+      const lessonLength = usedLesson.length || 1;
+      delete usedLesson.length;
+      for (var i = 0; i < lessonLength; i++) parsedDay.push(usedLesson);
+    }
+    return parsedDay;
+  });
+});
+
+export const possibleTimetableGroups = computed(() => {
+  const foundTimetableGroups = [];
+  if (!timetable.value.data) return [];
+  // Make a list of all possible timetable groups
+  // found in the current timetable
+  for (const day of timetable.value.data) {
+    for (const lesson of day) {
+      if (Array.isArray(lesson)) {
+        for (const group of lesson) {
+          if (!foundTimetableGroups.includes(group.group)) {
+            foundTimetableGroups.push(group.group);
+          }
+        }
+      }
+    }
+  }
+  return foundTimetableGroups;
+});
+
+// Initially fetch data for the
+// current, next and previous day
+fetchData(getNextAndPrevDay(selectedDate.value), false);
diff --git a/src/util.js b/src/util.js
index cba94c5..2bce7b3 100644
--- a/src/util.js
+++ b/src/util.js
@@ -33,3 +33,11 @@ export function getDateSkippingWeekend(date, onlyNext) {
     );
   return date;
 }
+
+export function getNextAndPrevDay(date) {
+  return [
+    new Date(date.getTime()),
+    getDateSkippingWeekend(new Date(date.getTime() + 86400000)),
+    getDateSkippingWeekend(new Date(date.getTime() - 86400000)),
+  ].map((e) => e.getTime());
+}