The criteria for finding Mondays would be:

WEEKDAY(date_cell)= 2

as WEEKDAY returns 1 for Sunday, 2 for Monday etc.

You could use an array formula instead of SUMPRODUCT, along the lines
of:

=SUM(IF((criterion1)*(criterion2)*(criterion3),ran ge_to_sum,0))

but it will be about as long as the SUMPRODUCT formula and will have to
be entered with CTRL-SHIFT-ENTER.

Hope this helps.

Pete