The traditional Jailbreak event was the "Sunday Lockdown", every Sunday evening, euro time. We also slipped in the "Wednesday Parole" some time after JB200x was released to have dedicated events for UTJB3 and JB200x, and both events were well-attended for a long time. Unfortunately they both no longer get much attention.
I'd vote for Saturday as well, if possible at a time that works for both US and euro people, i.e. it shouldn't start much later than, say, 8pm UTC. (I think that was the original Lockdown/Parole time.) If players like the new event, we could probably add it as our new Lockdown countdown on PlanetJailbreak.