Unlike with contacts, there currently there are no date updated fields in the event portions of the API. Without knowing when the database was last changed regarding events, there does not seem to be an easy way for an outside application to know when it should be querying data for changes since the last time that events have changed.

The minimum would seem to be

Events Last Updated - across all events.
Event Registrations Last Updated - across all events
Event Last Updated - for a single event.
Event Registrations Last Updated - for a single event.

A couple more event level date fields might be useful:

Event Schedule Last Updated - this would track changes in event session times only, not other items.