This is perhaps the easiest step, but it takes careful consideration nonetheless. DATE and TIME are essential. Beware of holiday weekends when people might be out of town or hosting, and major sports events. Early Saturday morning might be a tough sell for those late night Friday revelers, and same goes with early Sunday morning for the church-goers. And remember that people will want to hang out and help out for probably no more than two hours.
Still unsure? Try a Saturday from about 10:30 to 12.
And of course, set a RAIN DATE or a rainy weather alternative activity. (Enlist strangers to clean out your basement!)