Formula to produce Sunday date

Hey all,

I'm try to figure out a formula that will calculate Sunday's date of a given week based on a Monday-Sunday date in a different cell.

EX. A1 - 5/12/11 result in A2 - 5/15/11
A1 - 5/15/11 result in A2 - 5/15/11

Any ideas?

Thanks Gang!