The Hebrew calendar is lunisolar. What that means in practice is that Passover can't fall in any other time of year than spring, which then anchors the rest of the festivals and other calendrical observances. The Hebrew calendar dates from the calculated date of the creation of the world, which corresponds to sunset on October 6 3761 BCE when you extend the traditional Julian calendar back in time before its implementation in 45 BCE.
Kara Goldfinch (dragonscave.space)