res/drawable/bg_day_selected.xml

<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item
        android:top="8dp"
        android:left="8dp"
        android:right="8dp"
        android:bottom="8dp">
        <shape android:shape="oval" >
            <size android:height="20dp" android:width="20dp" />
            <solid android:color="@color/accent_color" />
        </shape>
    </item>
</layer-list>