Normally Google is announcing the release of new games (as they did for the Farming Simulator Platinum edition for example), but most likely they'll post it at the moment in the social media channels like Twitter or Facebook.
They might also bring an announcement on https://blog.google, but I don't know if they would do that for every game they launch, probably only when they have released a bigger bunch of games.
Unfortunately the information is very rare from Google about upcoming events, which is a bit sad, because it would be better if we would know early when exactly a new game is available.