Sunday School classes and other small groups can concentrate their efforts to a weekend(s), but ultimately...if you can sign up to participate whenever it is convenient. If you're unavailable the week(s) your class wants to do a majority of the tasks you can sign-up to help another week. If sack lunches is your expertise, or you are an early riser and want to help clean up on Saturday mornings...don't feel like your class's weekend is the only time you can sign-up to help.