All are ancient symbols of sacrifice and fertility.
Fertility is the assurance of earthly continuance and comfort.
As represented by the sacrificial god, the cycling of the life-force back from the dark is cause for great celebration. In Jewish tradition the sacrificial lamb's blood on the doorposts assured life, literally, to slaves awaiting exodus from Egypt. The cherished tradition of Passover recalls the mercy and aspiration of new beginnings. The lily is another emblem of purity and the feminine principle often connected with the Virgin Mary and other mothers of the gods. The pig or boar was the sacred animal of the European goddess Eostre, hence the traditional Easter ham.