group into Week End dates

[Neal]

I trying assign a Week End or Week Beginning date to a
list of activities with a start and finish date column. I
would like generate a formula in a fourth column that
assigns a Week end date. After this, I can "countif" to
obtain the quantity of activities per week. I can not
figure out a formula to look at either the start or finish
date column and assign a Week end date. Any suggestions,
help, or directions?