Not sure about Long Meg on the 21st
However you probably want to avoid Castlerigg on 20th / 21st if there is a sizable gathering as there will already be a large number there
Parking gets tricky and police patrol the road
We stay in the lakes this time every year at this time
Depending on what sort of accommodation people want the travel lodge at Penrith is excellently placed to visit both Castlerigg and long meg (if its good enough for JC on the TMA video then who are we to argue!)