There are 2 types of events in MMORPGS.

Reoccurring Events: These events typically pop up on an annual or bi-annual basis. Most of the time, these types of events will coincide with some holiday, either national or international. They can also be used to celebrate world lore holidays and/or anniversaries (ones with no real-life counterparts). These types of events allow for reoccurring themes and awards.

Special Events: These types of events usually only happen once or twice in a MMO's entire lifespan. Typically, these are used for promotional purposes for new products, games, or with special anniversary milestones (5-year anniversary, 10-year anniversary, etc.). Once these events conclude, the content and awards are no longer obtainable.

Heavensturn, Moonfire Fair, The Rising, Starlight Celebration, etc. events fall into the former category.
The One Year Anniversary and the FFXIII, FFXI, and Dragon Quest X crossover events fall into the latter category.