Easter and Christmas traditions, like much of the Christian mythos, borrowed heavily from other religions. The name even comes from Eostre, a Germanic pagan goddess. As Christianity spread, it absorbed people of a variety of cultures both willingly and unwillingly. In some cases, whoever was actively trying to spread the religion would try to build common ground by including parts of the cultures it replaced. In other cases, people just maintained parts of their culture and blended it into their new religion over time.
It's hard to really pinpoint a date and say when the Christian tradition began since it consists of so many traditions rolled together. In general, Christianity was built on various other traditions and lore repackaged, not really new material.