Frequently, ArenaNet modifies the game environment or rules for short periods, in recognition of a real world event (such as a holiday or anniversary).
Types of Events
Holiday Events usually consist of changes to the ambient environment of major outposts, as well as unique quests, drops or rewards. Wintersday, Halloween, and the Dragon Festival are good examples of holiday events.
Preview Events or Beta Events give players previews of upcoming content or expansions, often allowing exploration of content that has yet to be finished by development. The Factions Preview Event is a good example of this style of event.
Weekend Events are often small changes to the rewards for certain actions, such as double Balthazar Faction, or free access to restricted content. ArenaNet often has events of this type, most weekends. Elite Weekend is a good example.