“And it shall be when the LORD shall bring thee into the land of the Canaanites, and the Hittites, and the Amorites, and the Hivites, and the Jebusites, which he sware unto thy fathers to give thee, a land flowing with milk and honey, that thou shalt keep this service in this month.”
King James Version (KJV)
13:5 When the lord shall bring you into the land, thou shalt keep this service - 'Till then they were not obliged to keep the passover, without a particular command from God.