Although there's nothing announced Inno do seem fairly settled on a rhythm of 3 week event, 1 week off, 1 week FA, 1 week off and repeat; with, for example, the spire rewards changing every 6 weeks to match
So some time the middle of next week would be a good guess
the last FAs have all started on a Thursday on live servers, so I would guess Thursday next week as this Thursday would not allow a 48h notice after the end of the current event...