At the most basic level, you want to do the following:
Add a new interaction on the timeline of a constituent who attended the event
Enter the Date as the date of the event
Subject should be something along the lines of attended the 2015 xyz Event
Purpose should be Special Event
Channel should be in Person
You might also have an interaction based custom field (Text/Pick One) for Event Name with values for each of your yearly events like Gala, Picnic, Fun Run, etc