On the "Events" tab, you’ll see the start and end dates of special events, tasks (or conditions) for these events, as well as possible rewards. 

By fulfilling these conditions, you’ll receive Marks of Excellence. This resource can then be exchanged in the "Rewards" tab.

The event system has nothing to do with the seasons, you can spend gained Marks at any time. However, do not forget that the "Rewards" tab is renewed regularly ;)