Start of Week

-- function to return the date of Monday in a week
CREATE Function StartOfWeek
(
@date datetime = null
)
RETURNS datetime
AS
BEGIN
DECLARE @Monday datetime

IF @date IS NULL
SET @date = GetDate()

-- round to midnight
SET @date = dateadd(dd,0, datediff(dd,0, @date))

SET @Monday = DateAdd(d, -((@@DATEFIRST + DatePart(dw, @date) -2) % 7), @date)

RETURN @Monday
END