Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 14454)
+++ wp-includes/functions.php	(working copy)
@@ -192,24 +192,15 @@
 	$md = substr( $mysqlstring, 5, 2 ); // Mysql string day
 	$day = mktime( 0, 0, 0, $md, $mm, $my ); // The timestamp for mysqlstring day.
 	$weekday = date( 'w', $day ); // The day of the week from the timestamp
-	$i = 86400; // One day
 	if ( !is_numeric($start_of_week) )
 		$start_of_week = get_option( 'start_of_week' );
 
 	if ( $weekday < $start_of_week )
-		$weekday = 7 - $start_of_week - $weekday;
+		$weekday += 7;
 
-	while ( $weekday > $start_of_week ) {
-		$weekday = date( 'w', $day );
-		if ( $weekday < $start_of_week )
-			$weekday = 7 - $start_of_week - $weekday;
-
-		$day -= 86400;
-		$i = 0;
-	}
-	$week['start'] = $day + 86400 - $i;
-	$week['end'] = $week['start'] + 604799;
-	return $week;
+	$start = $day - 86400 * ( $weekday - $start_of_week ); // The most recent week start day on or before $day
+	$end = $start + 604799; // $start + 7 days - 1 second
+	return compact( 'start', 'end' );
 }
 
 /**
@@ -4164,4 +4155,28 @@
 	@header( 'X-Content-Type-Options: nosniff' );
 }
 
+/**
+ * Returns a MySQL expression for selecting the week number based on the start_of_week option.
+ *
+ * @internal
+ * @since 3.0.0
+ * @param string $column
+ * @return string
+ */
+function _wp_mysql_week( $column ) {
+	switch ( $start_of_week = (int) get_option( 'start_of_week' ) ) {
+	default :
+	case 0 :
+		return "WEEK( $column, 0 )";
+	case 1 :
+		return "WEEK( $column, 1 )";
+	case 2 :
+	case 3 :
+	case 4 :
+	case 5 :
+	case 6 :
+		return "WEEK( DATE_SUB( $column, INTERVAL $start_of_week DAY ), 0 )";
+	}
+}
+
 ?>
Index: wp-includes/query.php
===================================================================
--- wp-includes/query.php	(revision 14454)
+++ wp-includes/query.php	(working copy)
@@ -1781,7 +1781,7 @@
 		}
 
 		if ( $q['w'] )
-			$where .= " AND WEEK($wpdb->posts.post_date, 1)='" . $q['w'] . "'";
+			$where .= ' AND ' . _wp_mysql_week( "`$wpdb->posts`.`post_date`" ) . " = '" . $q['w'] . "'";
 
 		if ( intval($q['comments_popup']) )
 			$q['p'] = absint($q['comments_popup']);
Index: wp-includes/general-template.php
===================================================================
--- wp-includes/general-template.php	(revision 14454)
+++ wp-includes/general-template.php	(working copy)
@@ -965,8 +965,8 @@
 			}
 		}
 	} elseif ( 'weekly' == $type ) {
-		$start_of_week = get_option('start_of_week');
-		$query = "SELECT DISTINCT WEEK(post_date, $start_of_week) AS `week`, YEAR(post_date) AS yr, DATE_FORMAT(post_date, '%Y-%m-%d') AS yyyymmdd, count(ID) as posts FROM $wpdb->posts $join $where GROUP BY WEEK(post_date, $start_of_week), YEAR(post_date) ORDER BY post_date DESC $limit";
+		$week = _wp_mysql_week( '`post_date`' );
+		$query = "SELECT DISTINCT $week AS `week`, YEAR( `post_date` ) AS `yr`, DATE_FORMAT( `post_date`, '%Y-%m-%d' ) AS `yyyymmdd`, count( `ID` ) AS `posts` FROM `$wpdb->posts` $join $where GROUP BY $week, YEAR( `post_date` ) ORDER BY `post_date` DESC $limit";
 		$key = md5($query);
 		$cache = wp_cache_get( 'wp_get_archives' , 'general');
 		if ( !isset( $cache[ $key ] ) ) {