This is the list of all the official events launched by the game staff.
On these occasions, players gain access to special maps, which usually pertains to specific battles and locations fought in World War 2, or at one time, included Arpeggio anime characters. These maps are only available on the duration of the event, and has usually better drop rates than normal sortie maps.
It is good to take note that we encounter special abyss fleet bosses which can only be seen on the said event, and are strong opponents. Though mainly depends on the map, recurring bosses such as the Battleship Princess on middle or pre-boss nodes make more players cry.
Starting Spring 2014, LOS 
-based checks were implemented that you total fleet LOS affects your movement through the map nodes, or just getting to the boss node. This then became common and spread even to extra operations maps.
Event List
Tips
- To join the events, the usually require 75% sortie win-rate. So watch your stats even before the event. Win-rates are reset every month.
- Stock-up on
Buckets and 
Steel. These two are the basic needs of your fleet during events because of repairs. Although other resources are also important depending on event (e.g. 
Bauxite for Midway).
- Some people say that natural regeneration of resources will allow you to finish all 5 maps for the duration of the event, usually two-weeks window. However, your fleet's consumption is still under the mercy of RNG, and how hard your ships get hit, and how many retries you take. It is better to be on the safe side.
- The natural regeneration tip is not applicable for buckets. Please do stock-up before the event.
- If the branching rule is difficult for the map, you might also find your ships feasting on your
Fuel reserves.
- Although a global game rule-of-thumb, ready/level at least two of each ship type because we'll never know the fleet composition requirements of the event maps.