wp-includes\general-template.php 

function get_calendar( $initial = true, $echo = true ) {
	global $wpdb, $m, $monthnum, $year, $wp_locale, $posts;

	$key   = md5( $m . $monthnum . $year );
	$cache = wp_cache_get( 'get_calendar', 'calendar' );

	if ( $cache && is_array( $cache ) && isset( $cache[ $key ] ) ) {
		/** This filter is documented in wp-includes/general-template.php */
		$output = apply_filters( 'get_calendar', $cache[ $key ] );

		if ( $echo ) {
			echo $output;
			return;
		}

		return $output;
	}

	if ( ! is_array( $cache ) ) {
		$cache = array();
	}

	// Quick check. If we have no posts at all, abort!
	if ( ! $posts ) {
		$gotsome = $wpdb->get_var( "SELECT 1 as test FROM $wpdb->posts WHERE post_type = 'post' AND post_status = 'publish' LIMIT 1" );
		if ( ! $gotsome ) {
			$cache[ $key ] = '';
			wp_cache_set( 'get_calendar', $cache, 'calendar' );
			return;
		}
	}

	if ( isset( $_GET['w'] ) ) {
		$w = (int) $_GET['w'];
	}
	// week_begins = 0 stands for Sunday.
	$week_begins = (int) get_option( 'start_of_week' );

	// Let's figure out when we are.
	if ( ! empty( $monthnum ) && ! empty( $year ) ) {
		$thismonth = zeroise( intval( $monthnum ), 2 );
		$thisyear  = (int) $year;
	} elseif ( ! empty( $w ) ) {
		// We need to get the month from MySQL.
		$thisyear = (int) substr( $m, 0, 4 );
		// It seems MySQL's weeks disagree with PHP's.
		$d         = ( ( $w - 1 ) * 7 ) + 6;
		$thismonth = $wpdb->get_var( "SELECT DATE_FORMAT((DATE_ADD('{$thisyear}0101', INTERVAL $d DAY) ), '%m')" );
	} elseif ( ! empty( $m ) ) {
		$thisyear = (int) substr( $m, 0, 4 );
		if ( strlen( $m ) < 6 ) {
			$thismonth = '01';
		} else {
			$thismonth = zeroise( (int) substr( $m, 4, 2 ), 2 );
		}
	} else {
		$thisyear  = current_time( 'Y' );
		$thismonth = current_time( 'm' );
	}

	$unixmonth = mktime( 0, 0, 0, $thismonth, 1, $thisyear );
	$last_day  = gmdate( 't', $unixmonth );
	$oooothisyear  = date( 'Y', $unixmonth  );
	$oooolast_day  = date( 't', $unixmonth );
	$oooothismonth = date( 'm' , $unixmonth);

	// Get the next and previous month and year with at least one post.
	$previous = $wpdb->get_row(
		"SELECT MONTH(post_date) AS month, YEAR(post_date) AS year
		FROM $wpdb->posts
		WHERE post_date < '$thisyear-$thismonth-01'
		AND post_type = 'post' AND post_status = 'publish'
			ORDER BY post_date DESC
			LIMIT 1"
	);
	$next     = $wpdb->get_row(
		"SELECT MONTH(post_date) AS month, YEAR(post_date) AS year
		FROM $wpdb->posts
		WHERE post_date > '$oooothisyear-$oooothismonth-{$oooolast_day} 23:59:59'
		AND post_type = 'post' AND post_status = 'publish'
			ORDER BY post_date ASC
			LIMIT 1"
	);

	/* translators: Calendar caption: 1: Month name, 2: 4-digit year. */
	$calendar_caption = _x( '%1$s %2$s', 'calendar caption' );
	$calendar_output  = '<table id="wp-calendar" class="wp-calendar-table">
	<caption>' . sprintf(
		$calendar_caption,
		$wp_locale->get_month( $thismonth ),
		gmdate( 'Y', $unixmonth )
	) . '</caption>
	<thead>
	<tr>';

	$myweek = array();

	for ( $wdcount = 0; $wdcount <= 6; $wdcount++ ) {
		$myweek[] = $wp_locale->get_weekday( ( $wdcount + $week_begins ) % 7 );
	}

	foreach ( $myweek as $wd ) {
		$day_name         = $initial ? $wp_locale->get_weekday_initial( $wd ) : $wp_locale->get_weekday_abbrev( $wd );
		$wd               = esc_attr( $wd );
		$calendar_output .= "\n\t\t<th scope=\"col\" title=\"$wd\">$day_name</th>";
	}

	$calendar_output .= '
	</tr>
	</thead>
	<tbody>
	<tr>';

	$daywithpost = array();

	// Get days with posts.
	$dayswithposts = $wpdb->get_results(
		"SELECT DISTINCT DAYOFMONTH(post_date)
		FROM $wpdb->posts WHERE post_date >= '{$thisyear}-{$thismonth}-01 00:00:00'
		AND post_type = 'post' AND post_status = 'publish'
		AND post_date <= '{$oooothisyear}-{$oooothismonth}-{$oooolast_day} 23:59:59'",
		ARRAY_N
	);

	if ( $dayswithposts ) {
		foreach ( (array) $dayswithposts as $daywith ) {
			$daywithpost[] = (int) $daywith[0];
		}
	}

	// See how much we should pad in the beginning.
	$pad = calendar_week_mod( gmdate( 'w', $unixmonth ) - $week_begins );
	if ( 0 != $pad ) {
		$calendar_output .= "\n\t\t" . '<td colspan="' . esc_attr( $pad ) . '" class="pad">?</td>';
	}

	$newrow      = false;
	$daysinmonth = (int) gmdate( 't', $unixmonth );
	$oooodaysinmonth = (int) date( 't', $unixmonth );

	for ( $day = 1; $day <= $oooodaysinmonth; ++$day ) {
		if ( isset( $newrow ) && $newrow ) {
			$calendar_output .= "\n\t</tr>\n\t<tr>\n\t\t";
		}
		$newrow = false;

		if ( current_time( 'j' ) == $day &&
			current_time( 'm' ) == $thismonth &&
			current_time( 'Y' ) == $thisyear ) {
			$calendar_output .= '<td id="today">';
		} else {
			$calendar_output .= '<td>';
		}

		if ( in_array( $day, $daywithpost, true ) ) {
			// Any posts today?
			$date_format = gmdate( _x( 'F j, Y', 'daily archives date format' ), strtotime( "{$thisyear}-{$thismonth}-{$day}" ) );
			/* translators: Post calendar label. %s: Date. */
			$label            = sprintf( __( 'Posts published on %s' ), $date_format );
			$calendar_output .= sprintf(
				'<a href="%s" aria-label="%s">%s</a>',
				get_day_link( $thisyear, $thismonth, $day ),
				esc_attr( $label ),
				$day
			);
		} else {
			$calendar_output .= $day;
		}

		$calendar_output .= '</td>';

		if ( 6 == calendar_week_mod( gmdate( 'w', mktime( 0, 0, 0, $thismonth, $day, $thisyear ) ) - $week_begins ) ) {
			$newrow = true;
		}
	}

	$pad = 7 - calendar_week_mod( gmdate( 'w', mktime( 0, 0, 0, $thismonth, $day, $thisyear ) ) - $week_begins );
	if ( 0 != $pad && 7 != $pad ) {
		$calendar_output .= "\n\t\t" . '<td class="pad" colspan="' . esc_attr( $pad ) . '">?</td>';
	}

	$calendar_output .= "\n\t</tr>\n\t</tbody>";

	$calendar_output .= "\n\t</table>";

	$calendar_output .= '<nav aria-label="' . __( 'Previous and next months' ) . '" class="wp-calendar-nav">';

	if ( $previous ) {
		$calendar_output .= "\n\t\t" . '<span class="wp-calendar-nav-prev"><a href="' . get_month_link( $previous->year, $previous->month ) . '">? ' .
			$wp_locale->get_month_abbrev( $wp_locale->get_month( $previous->month ) ) .
		'</a></span>';
	} else {
		$calendar_output .= "\n\t\t" . '<span class="wp-calendar-nav-prev">?</span>';
	}

	$calendar_output .= "\n\t\t" . '<span class="pad">?</span>';

	if ( $next ) {
		$calendar_output .= "\n\t\t" . '<span class="wp-calendar-nav-next"><a href="' . get_month_link( $next->year, $next->month ) . '">' .
			$wp_locale->get_month_abbrev( $wp_locale->get_month( $next->month ) ) .
		' ?</a></span>';
	} else {
		$calendar_output .= "\n\t\t" . '<span class="wp-calendar-nav-next">?</span>';
	}

	$calendar_output .= '
	</nav>';

	$cache[ $key ] = $calendar_output;
	wp_cache_set( 'get_calendar', $cache, 'calendar' );

	if ( $echo ) {
		/**
		 * Filters the HTML calendar output.
		 *
		 * @since 3.0.0
		 *
		 * @param string $calendar_output HTML output of the calendar.
		 */
		echo apply_filters( 'get_calendar', $calendar_output );
		return;
	}
	/** This filter is documented in wp-includes/general-template.php */
	return apply_filters( 'get_calendar', $calendar_output );
}