To convert a year-week to a date, then you should also specify the weekday: : STR_TO_DATE « Date Time Functions « MySQL Tutorial

1. MySQL Tutorial
2. Date Time Functions
3. STR_TO_DATE

mysql>
mysql>
mysql> SELECT STR_TO_DATE('200442 Monday', '%X%V %W');
+-----------------------------------------+
| STR_TO_DATE('200442 Monday', '%X%V %W') |
+-----------------------------------------+
| 2004-10-18                              |
+-----------------------------------------+
1 row in set (0.00 sec)

mysql>