I know that the following will work
=SUMPRODUCT($F$5:$F$370,--(MOD($B$5:$B$370,7)=$S8))/SUMPRODUCT(--($F$5:$F$3700),--(MOD($B$5:$B$370,7)=$S8))

However
=SUMPRODUCT($F$5:$F$370,--(WEEKNUM($B$5:$B$370)=$S8))/SUMPRODUCT(--($F$5:$F$3700),--(WEEKNUM($B$5:$B$370)=$S8))

MOD will work but WEEKNUM Will not work - is there a list of functions that
will work (when dealings with vector/arrays) with sumproduct and ones that
will not?

Or do I have to do something special with the weeknum command to be able to
work on something other than one cell?