Mitterfruhl

At Spring Equinox or Mitterfruhl, peasants sacrifice a firstborn lamb to Taal and Rhya. Manann also receives sacrifice, so that he might calm the seas and spare the land from flooding rivers. The followers of Ulric ritually lament their god’s passing and his priests remain in mourning until the autumn.

A notable custom in Altdorf is the offering of a painted griffon’s egg to the Temple of Sigmar. Adventurers comb the mountains for a suitable egg in the preceding months. Few return, and only the largest egg is chosen. Military disaster is foretold for a year when no egg can be presented.

Source

 * Warhammer Fantasy RPG 3rd ED -- Signs of Faith pg. 30