Easter is an ancient terran holiday.
By 2500, the inhabitants of the Koprulu Sector did not know much about the holiday. Anthropologists working for the Kel-Morian Combine studied many historic holidays originating from Earth, and though they disagreed on what the meaning of Easter was, they were able to piece together how it was celebrated. A giant anthropomorphic badger travelled civilizations, gifting the people with SynthEggs crafted into the shape of birds and rabbits. The day then culminated with a great chrysalis hunt that involved the slaughter of warring factions. The anthropologists believed the latter part of the festivities is a more modern addition to the ancient tradition.
1999-04-02. Egg Madness. StarCraft Compendium. Accessed on 2015-12-12