Need data validation for Monday thru Thursday

[thegrahambo]

Application is project management.

I've searched around quite a bit and have not been able to figure out how to only allow calculated dates to a field that fall on a Monday through Thursday via data validation.

This is what I've got plugged in there now:
=WEEKDAY(H8:M15,2)<5

I have a start date, a duration defined in another cell and am using those two added together to determine a calculated end date which must be M-Th.
If the date is not M-Th, the date i'd like displayed is the next date that falls on a M-Th.