Sunday is 1 (1 or omitted): Sunday is the first day (day 1) of the week and Saturday is day 7.
Monday is 1 (2): Monday is the first day (day 1) of the week and Sunday is day 7.
Example
=WEEKNUM("12/7/2009",1) returns 29, if Sunday is considered the first day of the week.
=WEEKNUM("12/7/2009",2) returns 28, if Monday is considered.