Is it possible to pass the past festivals? (Assassin's Creed: Valhalla)

Not

You can, but you have to wait for their start date, they are repeated from year to year.
They are tied to dates, like events in other games. Like Halloween or Christmas.
In fact, these are events in honor of the holidays, only these holidays are purely Germanic peoples, the same Yule or Ostara.
I missed one myself, but then it happened again the following year and I successfully completed it.