iCalendar

QCalendarWidget *wCalendrier = new QCalendarWidget(this);

wCalendrier->setMinimumDate(QDate(1900, 1, 1));
wCalendrier->setMaximumDate(QDate(3000, 1, 1));
wCalendrier->setGridVisible(true);

QTextCharFormat format;
format.setFontWeight(QFont::Bold);
wCalendrier->setHeaderTextFormat(format);

connect(wCalendrier, SIGNAL(clicked(QDate)), this, SLOT(selectionnerDate(QDate)));

void MainWindow::selectionnerDate(QDate date)
{
    qDebug() << __FUNCTION__ << date;
}

QNetworkAccessManager networkManager;
QNetworkRequest requete;
QNetworkReply* networkReply;

QUrl hostURL("https://calendar.google.com/calendar/ical/u7m67cnmhd7b50q9qjndnus0ag%40group.calendar.google.com/public/basic.ics");

requete.setUrl(hostURL);
requete.setRawHeader("User-Agent", "CalendarClient_ICS");
requete.setRawHeader("Depth", "0");
requete.setRawHeader("Prefer", "return-minimal");
requete.setRawHeader("Content-Type", "text/xml; charset=utf-8");
requete.setRawHeader("Content-Length", "0");

QSslConfiguration configuration = requete.sslConfiguration();
configuration.setPeerVerifyMode(QSslSocket::VerifyNone);
requete.setSslConfiguration(configuration);

networkReply = networkManager.get(requete);

if (networkReply != nullptr)
{
  connect(networkReply, SIGNAL(error(QNetworkReply::NetworkError)), this, SLOT(handleHTTPError()));
  connect(networkReply, SIGNAL(finished()), this, SLOT(traiterRequete()));
}

void Calendrier::traiterRequete()
{
    if (networkReply != nullptr)
    {
        QBuffer buffer(this);
        buffer.setData(networkReply->readAll());

        buffer.open(QIODevice::ReadOnly);
        bool trouve = false;
        do
        {
            QString ligne = buffer.readLine();
            if (!ligne.isNull())
            {
                if (ligne.startsWith("X-WR-CALNAME"))
                {
                    QString nomCalendrier = ligne.trimmed();
                    nomCalendrier.remove(";LANGUAGE=fr", Qt::CaseInsensitive);
                    nomCalendrier.remove("X-WR-CALNAME:", Qt::CaseInsensitive);
                    qDebug() << __FUNCTION__ << nom << nomCalendrier;
                    trouve = true;
                }
            }
        }
        while (!buffer.atEnd() && !trouve);
        buffer.seek(0);
        buffer.close();

        contenuCalendrier = new QTextStream(buffer.data());

        // ...
    }
}

...
<CalendarView
    android:id="@+id/calendarView"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginTop="2dp"
    android:layout_marginBottom="15dp" />
...

public class MainActivity extends AppCompatActivity
{
  private CalendarView calendarView = null;
  final int LUNDI = 2; // Premier jour de la semaine !

  @Override
  protected void onCreate(Bundle savedInstanceState)
  {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);

      calendarView = (CalendarView) findViewById(R.id.calendarView);
      calendarView.setDate(System.currentTimeMillis(), false, true); // aujourd'hui
      calendarView.setFirstDayOfWeek(LUNDI); // Premier jour de la semaine
      calendarView.setOnDateChangeListener(new CalendarView.OnDateChangeListener()
      {
          @Override
          public void onSelectedDayChange(CalendarView view, int year, int month, int dayOfMonth)
          {
            Log.d(TAG, "onSelectedDayChange() selectedDate = " + dayOfMonth + "/" + (month + 1) + "/" + year);
          }
      });
  }
}

plugins {
    id 'com.android.application'
}

android {
    compileSdkVersion 30
    buildToolsVersion "30.0.3"
    defaultConfig {
        applicationId "com.lasalle.calendrier"
        minSdkVersion 24
        targetSdkVersion 30
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
}

dependencies {
    implementation 'org.mnode.ical4j:ical4j:1.0.5'
    implementation 'backport-util-concurrent:backport-util-concurrent:3.1'
    implementation 'commons-codec:commons-codec:1.8'
    implementation 'commons-lang:commons-lang:2.6'

    implementation 'androidx.appcompat:appcompat:1.2.0'
    implementation 'com.google.android.material:material:1.3.0'
    implementation 'androidx.constraintlayout:constraintlayout:2.0.4'
    testImplementation 'junit:junit:4.+'
    androidTestImplementation 'androidx.test.ext:junit:1.1.2'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.3.0'
}

// Parsing a calendar file
FileInputStream fin = new FileInputStream("mycalendar.ics");
CalendarBuilder builder = new CalendarBuilder();
Calendar calendar = builder.build(fin);

// Parsing a calendar string
String myCalendarString = ...
StringReader sin = new StringReader(myCalendarString);
CalendarBuilder builder = new CalendarBuilder();
Calendar calendar = builder.build(sin);

// À partir d'une URL
URL url = new URL("https://calendar.google.com/calendar/ical/u7m67cnmhd7b50q9qjndnus0ag%40group.calendar.google.com/public/basic.ics");
HttpsURLConnection connexionHTTPS = (HttpsURLConnection) url.openConnection();
InputStream in = new BufferedInputStream(connexionHTTPS.getInputStream());
BufferedReader r = new BufferedReader(new InputStreamReader(in));
StringBuilder sb = new StringBuilder();
String ligne;
while(true)
{
    if (!((ligne = r.readLine()) != null))
        break;
    sb.append(ligne);
    sb.append("\r\n");
}
StringReader sin = new StringReader(contenuCalendrier);
calendar = builder.build(sin);

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.lasalle.calendrier">

    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.Calendrier">
        <activity android:name=".ParametresAgenda"></activity>
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

public class AsyncTaskHTTP extends AsyncTask<String, Void, String>
{
    public AsyncTaskHTTP()
    {
    }

    @Override
    protected String doInBackground(String... strings)
    {
      // Récupération du contenu du calendrier .ics
    }

    @Override
    protected void onPostExecute(String contenuCalendrier)
    {
      // Création du calendrier
    }
}

// Le contenu du calendrier
Log.d(TAG, "Agenda = " + calendar);

// Quelques les propriétés du calendrier
Log.d(TAG, "ProductId = " + calendar.getProductId().toString());
Log.d(TAG, "Version = " + calendar.getVersion().toString());
Log.d(TAG, "CALNAME = " + calendar.getProperty("X-WR-CALNAME").getValue());

// Toutes les propriétés du calendrier
PropertyList plist = calendar.getProperties();
for (Object object : plist.toArray())
{
    Log.d(TAG, "objet = " + object);
}

TimeZone tz = TimeZone.getTimeZone("France/Paris");
java.util.Calendar dateDalendrier = new GregorianCalendar(annee,mois,jour,heure,minute,0);
dateDalendrier.setTimeZone(tz);

java.util.Calendar dateDalendrierFin = new GregorianCalendar(annee,mois,jour,23,59,59);
dateDalendrierFin.setTimeZone(tz);

// choix du format
SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy HH:mm", Locale.FRENCH);
sdf.setTimeZone(TimeZone.getTimeZone("UTC"));
String jourChoisi = sdf.format(dateDalendrier.getTime()); // à partir du timestamp

List evenements = null; // la liste des évènements du jour choisi
if (calendar != null && chargement)
{
    String jourFin = sdf.format(dateDalendrierFin.getTime()); // à partir du timestamp
    DateTime debut = null;
    DateTime fin = null;
    try
    {
        debut = new DateTime(dateDalendrier.getTime()); // le jour choisi
        fin = new DateTime(dateDalendrierFin.getTime().getTime());
    }
    catch (Exception e)
    {
        e.printStackTrace();
    }

    // Définit la période d'une journée
    Period periode = new Period(debut, fin);

    // Crée un filtre pour cette période
    Filter filtre = new Filter(new PeriodRule(periode));

    // Filtre les évènements du jour choisi
    evenements = (List) filtre.filter(calendar.getComponents(Component.VEVENT));

    // Déboguage
    for (Object obj : evenements)
    {
        VEvent evenement = (VEvent) obj;

        // Toutes les propriétés de l'évènement
        for (Object o : event.getProperties())
        {
            Property property = (Property) o;
            Log.d(TAG, "(VEvent) Property [" + property.getName() + ", " + property.getValue() + "]");
        }
    }
}