Update 21, which was just rolled out, was done to launch the Atlantis Rises event. details can be found here:
Basically it is expected that near the end of each month (I think people are expecting the 4th thursday, so 3 days from this post) the Atlantis summons will come back (with a couple new heroes), all Atlantis stages will cost a bit less world energy, and loot from Atlantis stages will be increased.
So it sounds like it will be a new monthly event focused around season 2. We’ll have to wait a few months before we can identify the exact timing pattern.