Ok, you can add the last two icons by changing WC-forecast.php 'NO' => array( array(-1,1,3,5,7), array(0,2,4,6,8) ), // Night first icon, Day over Night
'DO' => array( array(0,2,4,6,8), array(1,3,5,7,9) ) // Day First icon, Day over Night
to 'NO' => array( array(-1,1,3,5,7,9), array(0,2,4,6,8,10) ), // Night first icon, Day over Night
'DO' => array( array(0,2,4,6,8,10), array(1,3,5,7,9,11) ) // Day First icon, Day over Night
and the last day available will show icons in Day-over-Night display mode.