Considering the devs stated that Palpatine and Yoda were going to be on a rotating schedule back in March / April and that Palpatine is scheduled for later this month, I stated October for the return. Now this could change if they (the devs) decide to incorporate the R2 and / or the Thrawn events into that rotation which they may do at some point.
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When in doubt, check the Events Calendar.
And if it's not listed, nobody knows for sure.
It's going to be November now. Every time someone makes a thread like this they add on a month.