The devs have set times to release brand new game modes/maps, characters, etc every 3 months roughly. But they will always have holidays and events in between. Just like valentines day, Halloween and Christmas, I think these are events that don't land on their scheduled quarterly releases but are bound to revisit the game in some form. I personally think summer will be celebrated again, or at least I hope. The summer themed skin for Cthulu is already a testament to that. It's just unconfirmed if they'll be the same skins and all of them at that.
