@ubi-viral I still have this issue and the is updated to the last update.

@ubi-viral I dont understand something.
my girlfriend is playing on PS 5, she updated the game and the Ostara Festival didnt show up, but after about an hour it started to work.
and I updated my game on PC to update 1.5.1 and the Ostara Festival didnt show up, I restarted the game and still didnt work. I deleted the game and download and it still didnt show up. I checked the game filles and they are fine. even when I swich to PS5 the Ostara Festival does not show up. I do have the mission to start the Festival, even the time that is left for the even shows up but no Festival.

The world event does not show up
I tried to restart the load of the game.
I tried to restart the game (I mean to close the game and start it again)
I tried to re-instal the game.
nothing seems to help.