You would need to create an enterprise calendar called something like
Weekend Work Only. The calendar would have Saturday and Sunday set as
working days, with Monday through Friday set as nonworking days. Your PM's
would need to assign the Weekend Work as a Task Calendar to any task that
must occur on a weekend only. Hope this helps.
Assigning the task calendar is done on the Advanced tab of the Task
Information screen. You will also have to check the option "Scheduling
ignores resource calendars" if you want the resources assigned on the task
to be assigned on the weekend.