Is there any way of knowing which airports are being opened when the region changes come into affect later this evening?
Not really, you have to play it by ear.
IFATC controllers on expert server are volunteers, there is no roster and therefore no schedule. The currently open frequencies are always given at https://www.ifatc.org/
On the ATC schedule, sometimes there’s hubs. When the region change occurs, the hubs are going to be opened first. Whichever one of the other airports on the list for that day other than the hubs can be opened. (Randomly, not in order)
I will open from 18:00z at LESU today, that is an GA airport between France and Spain :-)!
If you are talking about Saturday and Sunday, then the airport with the most arrivals is definitely going to be open. Any other airport in the region can be opened, there’s not really a way of knowing which will be open.
Just gonna take a wild guess and say Heathrow and Amsterdam will be quite busy because no one can ever get enough of those airports. Personally, I’d suggest visiting Greece or the Alps region. You can find great scenery and some cool airports there :)