Ubaidullah bin Zhr عبيد الله بن زحر · The Book on Sacrifices

That the Messenger of Allah (ﷺ) said: "There is no Fara' or 'Atirah." The Fara' is the first of the offspring that would be born to them, so they would slaughter it. The 'Atirah was an animal that they would slaughter during Rajab to honor the month of Rajab, since it was the first of the sacred months.