An event cache marks the location, date, time, description, requirements, etc. of a geocaching event, which is an official gathering of geocachers. Geocaching events are great opportunities to meet other geocachers in your area.
Many geocaching event caches that are sponsored by a local Geocaching Club will have food, fun, and fraternizing. Friends and family are often invited to geocaching event caches, even if they do not participate in Geocaching. Thus, geocaching event caches are a great method to recruit new geocachers.
The first recorded geocaching get-together (now known as an event cache) was held in Austin, Texas, on March 24, 2001. It was hosted by Eoghan and Pumpkin Princess. The "hide" date was set as the actual date of the event (which became standard practice) and the meeting location coordinates were used for the "cache coordinates". (Ref: The History of Geocaching)
CITO event caches are events held specifically to clean up an area.