Ok, ok ok 
Still don't get proper result with date range 1899-12-29 and 1900-01-03.
Running
select datediff(d, '19000101', '1900-01-03')/7 - datediff(d, '19000102', '1899-12-29')/7
gives me 0 mondays. Shouldn't it report 1 monday for January 1, 1900?
N 56°04'39.16"
E 12°55'05.25"