Try these:

If the weeknumber is in A1, then:

=DATE(2013,1,A1*7-2)-WEEKDAY(DATE(2013,1,3)) for Monday
=DATE(2013,1,A1*7-2)-WEEKDAY(DATE(2013,1,3)) + 2 for Wednesday
=DATE(2013,1,A1*7-2)-WEEKDAY(DATE(2013,1,3)) + 4 for Friday