ImageButton Calendar = (ImageButton) findViewById (R.id.Calendar);
Calendar.setOnClickListener(new View.OnClickListener() {

    @Override
    public void onClick(View v) {
        // TODO Auto-generated method stub

        Intent OpenCalendarbtn = new Intent (Menu.this, DisplayCalendar.class);
        startActivity (OpenCalendarbtn);
    }
});

<ImageButton
    android:id="@+id/Read"
    android:layout_width="100dp"
    android:layout_height="100dp"
    android:layout_alignParentRight="true"
    android:layout_alignTop="@+id/Write"
    android:layout_marginRight="31dp"
    android:src="@drawable/read" />

<ImageView
    android:id="@+id/AppLogo"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentTop="true"
    android:layout_centerHorizontal="true"
    android:src="@drawable/logo" />

<ImageButton
    android:id="@+id/Write"
    android:layout_width="100dp"
    android:layout_height="100dp"
    android:layout_alignParentLeft="true"
    android:layout_below="@+id/AppLogo"
    android:layout_marginLeft="35dp"
    android:layout_marginTop="14dp"
    android:src="@drawable/write" />

<ImageButton
    android:id="@+id/Calendar"
    android:layout_width="100dp"
    android:layout_height="100dp"
    android:layout_alignLeft="@+id/Write"
    android:layout_below="@+id/Write"
    android:layout_marginTop="20dp"
    android:src="@drawable/calendar/>

ImageButton Calendar = (ImageButton) findViewById (R.id.Calendar);

Button Calendar = (Button) findViewById (R.id.Calendar);