In a post on Steam, Amplitude Studios announced that the “Endless Day” is back, and Endless Space, Endless Legend, and Dungeon of the Endless will get free weekends to accompany it. Here is the breakdown from the Steam Page.
How to unlock the Emlek Tarosh hero: you need to start a new game of Endless Space (make sure you have disabled your mods if you are playing with any), before January 25th (end of the event). If you manage to do so, the hero will be unlocked for your future games once the event is over.
To unlock the “Endless Day” achievement: win a game in “Endless” difficulty, started between January 21st and January 25th, but completed at any time.
DUNGEON OF THE ENDLESS
How to unlock the archivist, Esseb Tarosh: you need to start a new game of Dungeon of the Endless. The hero will be available in your pool right away, for the duration of the event. Like any other heroes in DotE, you need to survive at least three floors with the hero or finish the game with it. If you manage to do so, once the event ends, you will be able to keep the Esseb Tarosh in your pool of heroes.
How to unlock the “Endless Day” achievement: you need to start a new game in any difficulty, hire the archivist hero and keep it alive, and complete the game. Make sure you start and complete a game between January 21st and January 25th in order to get the achievement.
How to unlock the Eslek Tarosh hero and the “Endless Day” achievement: you need to start a new game of Endless Legend (make sure you have disabled your mods if you are playing with any), before January 25th (end of the event!). Once you have built your first city, a new quest will appear: “Day of the Awakening”. Complete it and you will unlock the new hero and the Steam achievement.