Group Query Results by Start of Week - php

Question

I have a few different tables that I'm trying to join aggregate results out of and group them based on different time intervals.
calendar table
+------------+
| adate |
+------------+
| 2015-09-24 |
| 2015-09-25 |
| 2015-09-26 |
| 2015-09-27 |
| 2015-09-28 |
....etc.......
plays table
+----+------------+----------+---------------------+---------------------+
| id | session_id | video_id | created_at | updated_at |
+----+------------+----------+---------------------+---------------------+
| 1 | 1 | 1 | 2015-09-25 15:49:50 | 2015-09-25 15:49:50 |
+----+------------+----------+---------------------+---------------------+
And the PHP/SQL:
switch ( TRUE ) {
case ($interval->days <= 7):
default:
$group_by = 'DATE(dates.adate)';
break;
case in_array( $interval->days, range(8, 31) ):
$group_by = 'CONCAT(YEAR(dates.adate), "/", WEEK(dates.adate))';
break;
case ($interval->days > 31):
$group_by = 'YEAR(dates.adate), MONTH(dates.adate)';
break;
}
$sql = '
SELECT calendar.adate,
IFNULL(g.event_count, 0) event_count
FROM calendar
LEFT JOIN (
SELECT DATE(events.created_at) created_at,
COUNT(*) event_count
FROM plays events
WHERE events.video_id = %d
AND events.created_at >= %s
AND events.created_at < date_add(%s, INTERVAL 1 DAY)
GROUP BY DATE(events.created_at)
) g
ON g.created_at = calendar.adate
WHERE calendar.adate BETWEEN %s AND %s
GROUP BY ' . $group_by . '
ORDER BY adate;
';
I'd like to change the grouping based on the interval length, ie. if it's a week's worth of results then group by each day, a months worth of results should group by "week of 9/28/15", a years worth of results should group by mm/yyyy
Can you show how to group the result set accordingly? I feel like I am close but missing something here.