Legendary Heroes

Theyre available to be farmed when their events come back. Palp and Yoda have happened once a month at least i believe? Thrawn is making a return soon and Jedi Rey won't be back for another 3 and half months since she was just released.

Luke recently returned but probably wont be back for a few months either

When will the dark side get their "hero" events would be the better question

The Darkside needs a new legend, Jango. Mandatory Boba +4 BH.
Jango should have Bounty Hunter, Scoundrel and Separatist synergy.

yoda and palpatine are on a bi-monthly rotation (I believe yoda is near the end of this month again).
CLS is harder to tell since he just returned not to long ago after a ~4 months so my bet would be it will be another 4 before he returns again. I would expect about the same timeframe for rey.

so expect for rey around halfway april, cls near the end of april early may if they keep the same timetable

Moved to events.

In general, you can keep your eye on the event calendars for the current month: https://forums.galaxy-of-heroes.starwars.ea.com/discussion/150548/event-calendar-jan

You can see that Thrawn and Chimaera start the 18th, and Yoda starts the 25th. Palp rotates with Yoda, so he should be back next month.