A event normally refer to either Seasonal either Seasonal Event (SE) or Great Construction Era (GCE).
Seasonal Events are held every season. However there are two exceptions due to the incident between developers and publishers. Seasonal Event refers to (normally 4-5) clearing missions
Main page: Events/Seasonal_Events
Great Construction Era
Great Construction Eras are usually held between seasonal events. Rather than a mission, GCE is a special offer, probability of getting ships would be doubled to encourage admirals spending their resources and having fun
(sometimes it is not fun at all)
Main page: Events/Great_Construction_Era
Before a event, usually there are some time-limited quests to help admirals to prepare for the event. There are some festival-related quests, they are also time-limited.