The Age of the Four Fleets began when Captain George Hellman, after the devastating events of the Council Massacre, gathered the remaining ships and crew from the Alliance of Sky Sailors together at the Whispering Mountain.
This is time in which most of the series is based (beginning in book 3) and the entirety of both tv adaptions.
The Days of Broken Sky[]
The period of history after the Daggers destroyed The Aparatus, against the wishes of Amara Fate.
The Four Fleets took heavy losses and new captains stepped up to fill the roles of those who were lost in the great battle.
The nation of Skyfell shifts to become the primary antagonist, and the fleets need to find support outside the alliance to fight back.
This is also when the surface is explored more thoroughly, revealing more about The Before.
It begins halfway through Season 4 of the show.