Week Number in a Month

Here are 2 different ways, both are assuming the week starts on monday

If you want weeks to be whole, so they belong to the month in which they start: So saturday 2012-09-01 and sunday 2012-09-02 is week 4 and monday 2012-09-03 is week 1 use this:

declare @date datetime = '2012-09-01'
select datepart(day, datediff(day, 0, @date)/7 * 7)/7 + 1

If your weeks cut on monthchange so saturday 2012-09-01 and sunday 2012-09-02 is week 1 and monday 2012-09-03 is week 2 use this:

declare @date datetime = '2012-09-01'
select datediff(week, dateadd(week, datediff(week, 0, dateadd(month, datediff(month, 0, @date), 0)), 0), @date - 1) + 1