In this video it shows the steps to create your firebase database based Calendar App. This App can be used to store your reminders or events in your firebase database.

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Complete source code and other details:

package com.programmerworld.firebasecalendarapp;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.CalendarView;
import android.widget.EditText;

import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

public class MainActivity extends AppCompatActivity {

    private CalendarView calendarView;
    private EditText editText;
    private String stringDateSelected;
    private DatabaseReference databaseReference;

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

        calendarView = findViewById(R.id.calendarView);
        editText = findViewById(R.id.editText);

        calendarView.setOnDateChangeListener(new CalendarView.OnDateChangeListener() {
            @Override
            public void onSelectedDayChange(@NonNull CalendarView calendarView, int i, int i1, int i2) {
                stringDateSelected = Integer.toString(i) + Integer.toString(i1+1) + Integer.toString(i2);
                calendarClicked();
            }
        });
        databaseReference = FirebaseDatabase.getInstance().getReference("Calendar");
    }

    private void calendarClicked(){
        databaseReference.child(stringDateSelected).addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {
                if (snapshot.getValue() != null){
                    editText.setText(snapshot.getValue().toString());
                }else {
                    editText.setText("null");
                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {
            }
        });
    }

    public void buttonSaveEvent(View view){
        databaseReference.child(stringDateSelected).setValue(editText.getText().toString());
    }
}

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <CalendarView
        android:id="@+id/calendarView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="30dp"
        android:layout_marginTop="3dp"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/editText"
        android:layout_width="230dp"
        android:layout_height="63dp"
        android:layout_marginStart="21dp"
        android:layout_marginTop="17dp"
        android:ems="10"
        android:inputType="textPersonName"
        android:text="Name"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/calendarView" />

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="18dp"
        android:layout_marginTop="22dp"
        android:onClick="buttonSaveEvent"
        android:text="Save Event"
        app:layout_constraintStart_toEndOf="@+id/editText"
        app:layout_constraintTop_toBottomOf="@+id/calendarView" />

</androidx.constraintlayout.widget.ConstraintLayout>

// Top-level build file where you can add configuration options common to all sub-projects/modules.
buildscript {
    repositories {
        // Make sure that you have the following two repositories
        google()  // Google's Maven repository
        mavenCentral()  // Maven Central repository

    }
    dependencies {
        // Add the dependency for the Google services Gradle plugin
        classpath 'com.google.gms:google-services:4.3.13'

    }
}


plugins {
    id 'com.android.application' version '7.4.0' apply false
    id 'com.android.library' version '7.4.0' apply false
}

plugins {
    id 'com.android.application'
    id 'com.google.gms.google-services'
}

android {
    namespace 'com.programmerworld.firebasecalendarapp'
    compileSdk 33

    defaultConfig {
        applicationId "com.programmerworld.firebasecalendarapp"
        minSdk 33
        targetSdk 33
        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 'androidx.appcompat:appcompat:1.6.0'
    implementation 'com.google.android.material:material:1.7.0'
    implementation 'androidx.constraintlayout:constraintlayout:2.1.4'
    testImplementation 'junit:junit:4.13.2'
    androidTestImplementation 'androidx.test.ext:junit:1.1.5'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.5.1'

    implementation platform('com.google.firebase:firebase-bom:31.1.1')

    implementation 'com.google.firebase:firebase-firestore'
    implementation 'com.google.firebase:firebase-database'
    implementation 'com.google.firebase:firebase-messaging'

}

pluginManagement {
    repositories {
        google()
        mavenCentral()
        gradlePluginPortal()
    }
}
dependencyResolutionManagement {
    repositoriesMode.set(RepositoriesMode.FAIL_ON_PROJECT_REPOS)
    repositories {
        google()
        mavenCentral()
    }
}
rootProject.name = "Firebase Calendar App"
include ':app'