There doesn’t appear to be a built-in way — without templating — to determine “does a calendar event exist today” as a condition of an Automation.
Let’s take an example that is probably all-too common: an Automation should run to turn on holiday lights when triggered, but only if there is an all-day calendar event named “Holiday light time” in the local “Device automation” calendar that indicates today is in “holiday light time.”
There are various ways to approach this problem. Many of the ideas I’ve seen over the years have merit. Home Assistant now has a local Calendar. It provides a reasonable and well-understood model for managing some kinds of time-based “stuff.” People understand calendars.
Some kinds of Automations — especially seasonal ones — would very much benefit from being conditioned by the presence (or absence) of an event on a calendar.