We call them Working Parties. And - as an independent site - it's in our Terms & Conditions that all members are expected to help. (Which doesn't mean all the members all the time, but they all have a commitment to help in some way.)
We had our first one yesterday morning. We're renewing the central drainage trench through the centre of the site, so it was hard - and very messy - labour, with one group doing the actual digging, others barrowing away the spoil and others cutting through bramble ready for next week. We had something like a dozen folk turn up. We've got three more sessions this month.
But later in the year we'll have a couple of sessions cutting back the hedge along the road boundary, a litter pick and a general tidy-up day (weekend last year, when we had a skip). And whatever else needs doing.
As soon as members turn up once, they realise: a) what fun it is (seriously!), and b) what a difference it makes to the site have loads of people tackling one thing.