"give the month number of monday's date -1 "
Do you want to actually the month number of Sunday (Month(Monday's date -1)), or do you want to range your month numbers from 0 to 11 ((month number of Monday's date)-1) ?
Maybe like this (assuming weekstart is defined as Monday):
Month( WeekStart( DateField) -1) AS MonthField
Month(WeekStart(DateField))-1 AS MonthField