activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:custom="http://schemas.android.com/apk/res-auto"
    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=".CustomCalendarActivity">

    <com.bendriss.customcalendar.CustomCalendar
        android:id="@+id/singleCalendar"
        custom:layout_constraintStart_toStartOf="parent"
        custom:layout_constraintEnd_toEndOf="parent"
        custom:layout_constraintTop_toTopOf="parent"
        custom:layout_constraintBottom_toBottomOf="parent"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />


    <com.bendriss.customcalendar.DoubleCustomCalendar
        android:id="@+id/doubleCalendar"
        android:visibility="invisible"
        custom:layout_constraintStart_toStartOf="parent"
        custom:layout_constraintEnd_toEndOf="parent"
        custom:layout_constraintTop_toTopOf="parent"
        custom:layout_constraintBottom_toBottomOf="parent"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />


    <Button
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        custom:layout_constraintStart_toStartOf="parent"
        custom:layout_constraintEnd_toEndOf="parent"
        custom:layout_constraintBottom_toBottomOf="parent"
        android:layout_margin="16dp"
        android:id="@+id/change"
        android:text="Change Calendar"
        />

</android.support.constraint.ConstraintLayout>