alter function [dbo].[F_START_OF_WEEK] 
(
@DATE datetime,
-- Sun = 1, Mon = 2, Tue = 3, Wed = 4
-- Thu = 5, Fri = 6, Sat = 7
-- Default to Sunday
@WEEK_START_DAY int = 1 
) 
/*
Find the first date on or before @DATE that matches 
day of week of @WEEK_START_DAY.
*/
returns datetime
with schemabinding
as
begin
declare @START_OF_WEEK_DATE datetime
declare @FIRST_BOW datetime

-- Check for valid day of week
if @WEEK_START_DAY between 1 and 7
begin
-- Find first day on or after 1753/1/1 (-53690)
-- matching day of week of @WEEK_START_DAY
-- 1753/1/1 is earliest possible SQL Server date.
select @FIRST_BOW = convert(datetime,-53690+((@WEEK_START_DAY+5)%7))
-- Verify beginning of week not before 1753/1/1
if @DATE >= @FIRST_BOW
begin
select @START_OF_WEEK_DATE = 
dateadd(dd,(datediff(dd,@FIRST_BOW,@DATE)/7)*7,@FIRST_BOW)
end
end

return @START_OF_WEEK_DATE

end