Mitterfruhl - The Spring Equinox

The Spring Equinox or Mitterfruhl is the day on the Imperial Calendar when Ulric the God of Winter finally relinquishes his control of the world to his brother Taal the God of Nature and his wife Rhya the Earth Mother.
The transfer of power is recognised by farmers and peasants by the sacrifice a firstborn lamb to Taal and Rhya. Manann their son also receives a sacrifice in the hope that he will not become angry but will favour the world with fair weather, calm the seas and spare the land from storms, and flooding rivers that will damage the future harvest.
The followers of Ulric ritually lament their god’s passing and his priests remain in mourning until the autumn.
A notable but more dubious 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. Given that there are no griffons in the mountains and that there are no mountains anywhere near Altdorf it seems more likely that this is just the Sigmarite Church trying to muscle in on the popularity of the festival and provide rich Altdorfers something to waste their money on in order to swell the cult's coffers. It's quite possible that the eggs themselves are commissioned and sold by Sigmarite agents who pay a hefty commission to the church for the privilege.

Urban Celebrations

Urban celebrations of Mitterfruhl usually involve public festivities, dancing, fairs and music. Theoretically, the day is a public holiday and so all shops and traders should be closed, but in practice, the attraction of huge numbers of people to the locale usually results in a similar increase in commercial activity both lawful and unlawful to exploit the crowds. Stalls pay dearly for licenses to sell street food or souvenirs whilst pick-pockets and cut-purses prey on the unwary having paid heavy for the privilege of working the crowds to the local thieves guild.

Rural Celebration

Rural celebrations are more traditional with the ritual sacrifice of a lamb to the Gods followed by drinking, feasting, and dancing around a bonfire by scantily clad village maidens and young men wearing horned headdresses.  The smearing of bodies with earth and ash symbolises the unity between man and the soil whilst the ritual washing is supposed to symbolise the nurturing or the nature spirit.
Garlands of flowers and woven branches are paraded around the fires much like our own 'May Day' festivities and prayers are offered to Taal, Rhya and Manann for a bountiful harvest and fruitful year. The celebrations also usually feature the appearance of Taal and Rhya to bless the community and celebrate their rebirth and renewed love.