I'd like to request that the calendar app be enabled ... it's highly useful and I can see absolutely no drawback to enabling it.
Let's discuss. Enabling the app is easy, but maintaining another Calendar is another story. We have https://www.mediawiki.org/wiki/Project:Calendar and https://www.mediawiki.org/wiki/Events already, which have already duplication. Before adding a third we need to consider what is the goal we should aim for.