Event Tags are a great way of organizing and filtering events in your calendar. If you want to make sure that every event in your calendar has an Event Tag, there is a feature that you can turn on in the "Customize" screen for your calendar. When this feature is turned on, you or anyone with Edit Rights to your calendar won't be able to add an event to the calendar unless an Event Tag has been added.

To turn on this feature for your calendar follow the steps in the image below:

Once this feature is turned on you will see this message if you try to add an event to your calendar without adding an Event Tag: