Syntax:
WEEK(date[,mode])

This function returns the week number for date. The two-argument form
of WEEK() allows you to specify whether the week starts on Sunday or
Monday and whether the return value should be in the range from 0 to 53
or from 1 to 53. If the mode argument is omitted, the value of the
default_week_format system variable is used. See
http://dev.mysql.com/doc/refman/5.1/en/server-system-variables.html.

URL: http://dev.mysql.com/doc/refman/5.1/en/date-and-time-functions.html

