John Wesley on Exodus 12:2

This shall be to you the beginning of months - They had hitherto begun their year from the middle of September, but hence - forward they were to begin it from the middle of March, at least in all their ecclesiastical computations. We may suppose that while Moses was bringing the ten plagues upon the Egyptians, he was directing the Israelites to prepare for their departure at an hour's warning. Probably he had, by degrees, brought them near together from their dispersions, for they are here called the congregation of Israel; and to them, as a congregation, orders are here sent.