Event Calendars seem to be very common on the Drupal sites we build. One of the best ways of improving engagement on a site is to add content about the event after it happens. People who attended an event might come back for a recap, or to see pictures or notes from other participants, while people who did not attend can get a sense of what a future event might be like based on your past events.
A site visitor expects quite different things from a future event versus a past event: for future events, you want to know the details of how to attend. If it has already happened, you can't attend, so those details are not useful -- but a video of the event very well might be.
The Events, Conditions, and Actions (ECA) module, and creating a new view mode for your content type (which is in Drupal core already) makes it trivial to automatically change the entire layout of an individual event after it has happened.
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