How do I find the next saturday in MySQL

You can use WEEKDAY and DATE_ADD function to calculate the next weekday incoming.

Here what you have to do:

SELECT 
  DATE_ADD(NOW(),INTERVAL IF(WEEKDAY(NOW())>=5,
                             (6-WEEKDAY(NOW())),
                             (5-WEEKDAY(NOW()))) DAY);

The query meaning:

With DATE_ADD you will add an interval between the parameter NOW() and one condition assuming my weekday for Saturday is 5, you will have to evaluate if NOW() is greater than 5 or equal, you have to rest it with 6 (Sunday) and if NOW() is less than 5 you have to rest it 5 with the weekday of NOW().

Test:

mysql> SELECT NOW();
+---------------------+
| NOW()               |
+---------------------+
| 2015-07-22 07:51:33 |
+---------------------+
1 row in set (0.00 sec)

mysql> SELECT DATE_ADD(NOW(),INTERVAL IF(WEEKDAY(NOW())>=5,(6-WEEKDAY(NOW())),(5-WEEKDAY(NOW()))) DAY);
+------------------------------------------------------------------------------------------+
| DATE_ADD(NOW(),INTERVAL IF(WEEKDAY(NOW())>=5,(6-WEEKDAY(NOW())),(5-WEEKDAY(NOW()))) DAY) |
+------------------------------------------------------------------------------------------+
| 2015-07-25 07:51:34                                                                      |
+------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> 

EDIT:

If you need to calculate to get the next Saturday if the actual Saturday is at 9:30PM:

New syntax:

SET @ACTUAL_DATE=NOW();
SET @NEXT_SATURDAY=DATE_ADD(@ACTUAL_DATE,INTERVAL IF(WEEKDAY(@ACTUAL_DATE)=5 && TIME(@ACTUAL_DATE)>'21:30:00',
                                            7,
                                            IF(WEEKDAY(@ACTUAL_DATE)>=5,
                                                (6-WEEKDAY(@ACTUAL_DATE)),
                                                (5-WEEKDAY(@ACTUAL_DATE)))) 
                                        DAY);

Today test:

mysql> SET @ACTUAL_DATE=NOW();
Query OK, 0 rows affected (0.01 sec)

mysql> SET @NEXT_SATURDAY=DATE_ADD(@ACTUAL_DATE,INTERVAL IF(WEEKDAY(@ACTUAL_DATE)=5 && TIME(@ACTUAL_DATE)>'21:30:00',
    -> 7,
    -> IF(WEEKDAY(@ACTUAL_DATE)>=5,
    -> (6-WEEKDAY(@ACTUAL_DATE)),
    -> (5-WEEKDAY(@ACTUAL_DATE)))) 
    -> DAY);
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @NEXT_SATURDAY;
+---------------------+
| @NEXT_SATURDAY      |
+---------------------+
| 2015-07-25 21:30:01 |
+---------------------+
1 row in set (0.00 sec)

mysql> 

But if we're on July 25th at 21:30:01 (Saturday):

mysql> SET @ACTUAL_DATE='2015-07-25 21:30:01';
Query OK, 0 rows affected (0.00 sec)

mysql> SET @NEXT_SATURDAY=DATE_ADD(@ACTUAL_DATE,INTERVAL IF(WEEKDAY(@ACTUAL_DATE)=5 && TIME(@ACTUAL_DATE)>'21:30:00',
    -> 7,
    -> IF(WEEKDAY(@ACTUAL_DATE)>=5,
    -> (6-WEEKDAY(@ACTUAL_DATE)),
    -> (5-WEEKDAY(@ACTUAL_DATE)))) 
    -> DAY);
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @NEXT_SATURDAY;
+---------------------+
| @NEXT_SATURDAY      |
+---------------------+
| 2015-08-01 21:30:01 |
+---------------------+
1 row in set (0.00 sec)

I have another variation of the formula guaranteed to work (unless there is a power failure)

SET @now = NOW();
SET @nextsat = DATE(DATE(@now) + INTERVAL (5-WEEKDAY(@now)) DAY
    + INTERVAL 570 MINUTE
    + INTERVAL IF((HOUR(@now)*3600+MINUTE(@now)*60+SECOND(@now))>34200,
    IF(WEEKDAY(@now)=5,1,0),0) WEEK);

In case you are wondering, 09:30:00 AM is

• 570 Minutes after midnight
• 34200 Seconds after midnight

Let's test this formula out

RIGHT NOW

mysql> SET @now = NOW();
Query OK, 0 rows affected (0.00 sec)

mysql> SET @nextsat = DATE(DATE(@now) + INTERVAL (5-WEEKDAY(@now)) DAY
    ->     + INTERVAL 570 MINUTE
    ->     + INTERVAL IF((HOUR(@now)*3600+MINUTE(@now)*60+SECOND(@now))>34200,
    ->     IF(WEEKDAY(@now)=5,1,0),0) WEEK);
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @now,@nextsat;
+---------------------+------------+
| @now                | @nextsat   |
+---------------------+------------+
| 2015-07-22 11:53:33 | 2015-07-25 |
+---------------------+------------+
1 row in set (0.00 sec)

mysql>

SATURDAY 09:29:59 AM

mysql> SET @now = '2015-07-25 09:29:59';
Query OK, 0 rows affected (0.00 sec)

mysql> SET @nextsat = DATE(DATE(@now) + INTERVAL (5-WEEKDAY(@now)) DAY
    ->     + INTERVAL 570 MINUTE
    ->     + INTERVAL IF((HOUR(@now)*3600+MINUTE(@now)*60+SECOND(@now))>34200,
    ->     IF(WEEKDAY(@now)=5,1,0),0) WEEK);
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @now,@nextsat;
+---------------------+------------+
| @now                | @nextsat   |
+---------------------+------------+
| 2015-07-25 09:29:59 | 2015-07-25 |
+---------------------+------------+
1 row in set (0.00 sec)

mysql>

SATURDAY 09:30:00 AM

mysql> SET @now = '2015-07-25 09:30:00';
Query OK, 0 rows affected (0.00 sec)

mysql> SET @nextsat = DATE(DATE(@now) + INTERVAL (5-WEEKDAY(@now)) DAY
    ->     + INTERVAL 570 MINUTE
    ->     + INTERVAL IF((HOUR(@now)*3600+MINUTE(@now)*60+SECOND(@now))>34200,
    ->     IF(WEEKDAY(@now)=5,1,0),0) WEEK);
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @now,@nextsat;
+---------------------+------------+
| @now                | @nextsat   |
+---------------------+------------+
| 2015-07-25 09:30:00 | 2015-07-25 |
+---------------------+------------+
1 row in set (0.00 sec)

mysql>

SATURDAY 09:30:01 AM

mysql> SET @now = '2015-07-25 09:30:01';
Query OK, 0 rows affected (0.00 sec)

mysql> SET @nextsat = DATE(DATE(@now) + INTERVAL (5-WEEKDAY(@now)) DAY
    ->     + INTERVAL 570 MINUTE
    ->     + INTERVAL IF((HOUR(@now)*3600+MINUTE(@now)*60+SECOND(@now))>34200,
    ->     IF(WEEKDAY(@now)=5,1,0),0) WEEK);
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @now,@nextsat;
+---------------------+------------+
| @now                | @nextsat   |
+---------------------+------------+
| 2015-07-25 09:30:01 | 2015-08-01 |
+---------------------+------------+
1 row in set (0.00 sec)

mysql>

GIVE IT A TRY !!!