If Phoenix are getting reworked they're probably doing some extensive testing on the reworks/mechanics of each toon before release. That would make it foolish to announce the event before they know everything is in working order.
If they announced a date and found an issue with the rework everyone would roast them if they delayed it.
Not saying that's what is happening, but you would think a rework (possibly new character models too) to better reflect season 4 of rebels might take a bit of time and leave an uncertain timeframe.