matthewherbert

For this to work, your permalink structure cannot contain post name or ID. For some reason I couldn’t figure out, WP opens the link differently depending on the permalink structure. Here is my site so you can see how I made the adjustment… the calendar always corresponds to the current post:

http://www.thejunglecomic.com

Open archives-calendar-widget/arw-widget.php to edit. Copy the following code:

/***** MODIFIED BY The Jungle Comic *****/
/* BEGIN MODIFICATION */

// The Query
$args2 = array(
	'date_query' => array(
		array(
			'year'  => $months[$i]->year,
			'month' => $months[$i]->month,
			'day'   => $j
		),
	),
);
$the_query = new WP_Query( $args2 );

// The Loop
if ( $the_query->have_posts() ) {

	while ( $the_query->have_posts() ) {
		$the_query->the_post();
                       $postTitle = get_the_title();
                       $postID = get_the_ID();
	}

} else {
	// no posts found
}
/* Restore original Post Data */
wp_reset_postdata();

          $postYear = get_the_date('Y');
          $postMonth = get_the_date('n');
          $postDay =  get_the_date('j');

//Match calendar post dates with the currently displayed post

			if( $j == $postDay && $months[$i]->year == $postYear && $months[$i]->month == $postMonth )
				$spanID = "IsPost";
			else
				$spanID = "NotPost";

/* END MODIFICATION including below--->   

   Added ID="'.$spanID.'"  

*/

Paste right after:

if( $j == $todayDay && $months[$i]->year == $todayYear && $months[$i]->month == $todayMonth )
				$todayClass = " today";
			else
				$todayClass = "";

Immediately below the code you just pasted, add ID="'.$spanID.'"so that the existing code looks like this:

if(in_array ( $j , $dayswithposts ) )
				$cal .= '<span ID="'.$spanID.'" class="day'.$last.$todayClass.' has-posts"> <a>year, $months[$i]->month, $j )
                     .make_arcw_link($post_type, $cats).'>  '.$j.'</a></span>';
			else
				$cal .= '<span class="day'.$last.$todayClass.'">'.$j.'</span>';

Adjust style using CSS for #IsPost {