WP_Cal_Demo # 1

template-general.php

 <?php
  // Get days with posts
	$dayswithposts = $wpdb->get_results("SELECT DISTINCT DAYOFMONTH(post_date)
		FROM $wpdb->posts WHERE post_date >= '{$thisyear}-{$thismonth}-01 00:00:00'
		AND post_type = 'post' AND post_status = 'publish'
		AND post_date <= '{$thisyear}-{$thismonth}-{$last_day} 23:59:59'", ARRAY_N);
	
  if ( $dayswithposts ) {
		foreach ( (array) $dayswithposts as $daywith ) {
			$daywithpost[] = $daywith[0];
		}
	} else {
		$daywithpost = array();