Our early ancestors were deeply aware of changes in the seasons, and changes in the heavens above them. Indeed, our ancestors ignored such changes at their peril.
While they may not have known the physical dynamics or the astronomical term Vernal Equinox, as the days lengthened and grew warmer, the ancients were aware of a coming time of balance between dark and light.
Our early ancestors would be busy now, making final repairs to their plows, preparing their draft animals to pull the plows, checking their store of seed corn to plant. Their survival in the next year would depend much upon the corn seeds they were about to plant.
Thus, rituals that marked spring planting time celebrated the fertility of the earth, the viability of the seed, and entreated the gods and goddesses to ensure an abundant crop.
The ancient Saxons celebrated these rite-of-spring rituals with an uproarious festival, called by some Ostara, commemorating the goddess of rebirth and fertility, Eostre, and seeking her blessing upon the fields and crops.
When the second-century Christian missionaries encountered the Saxon tribes of the north, they attempted to convert the tribes to Christianity. These missionaries decided to spread their religious message slowly throughout the populations by allowing the people to continue to celebrate feasts of the old ways, but to do so in a Christian manner.
As the Christian celebration overtook the old ways, the goddess name, Eostre, eventually morphed to the modern word and spelling, Easter.
In A.D. 325, the Council of Nicaea was convened by emperor Constantine. It issued the Easter Rule which states that Christian Easter shall be celebrated on the first Sunday that occurs after the first full moon on or after the vernal equinox, further blending the ancient Rite-of-Spring celebration with Christian Easter.