As I understand it from those involved in the Hastings revival, it is not an attempt to replicate what used to happen (which died out at the end of the 19th century as a result of the prohibition of sending chidren up chimneys), but simply to have a jolly good dance event. So they hold it on the weekend nearest to May Day. This gives rise to the possibility of being able to make it to Padstow as well as Jack in the Green (or the Rochester Sweeps).