Not much to it. 'Bout 2000 years ago, Jesus got nailed to a cross because a bunch of people were dicks. Then he came back from the dead. So the Catholics and stuff go, "Yay, Jesus is back, let's boogie," and celebrate.
Meanwhile, up in other parts, a lot of people were having big shindigs about Spring coming back and having lots of sex to celebrate. It's life affirming, see, because all the trees and animals and shit are coming back. So they fuck.
And then the Christians and the other people got together and said, "Well, you got bunnies and birds, and we got Jesus, so let's put 'em together," because that's how Christians roll. So nowadays we celebrate Jesus' resurrection with bunnies and chicks. It probably made more sense at the time.