FESTIVAL:
The First of Nisan, the Forgotten Jewish New Year (Joel S. Davidi Weisberger, Jewish Link of New Jersey). It does look as though a spring New Year was once a thing in ancient Israel. But I didn't known about the Seder Al-Tawhid rite. Some somewhat related posts (on the Akitu, the Babylonian spring New Year festival) are
here and links.
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