today() - inserts today's date
weekday() - returns the day of the week in number format so
=weekday(today(),1) - uses Sunday = 1 through Saturday = 7
=weekday(today(),2) - uses Monday = 1 though Sunday = 7
=weekday(today(),3) - uses Monday = 0 through Sunday = 6
Below I'm using 1 for Monday but you can replace that number with whatever day you are trying to find. Using the same formula to find every Sunday, I would replace the bold 1 with a 7.
This finds the next Moday: