As the 10th plague was set to begin against the first-born children and livestock of Mitsraim (Egypt), God told Moshe (Moses) the month with Pesakh (Passover) and the Exodus would be the beginning of Yisra’el’s year (Ex. 12:2). God told Moses of the particular rituals that are to happen during this first month, called Aviv (Ex. 12:3–11). This was relayed to them at the beginning of the month, a couple of weeks before the final plague.
There are particular housekeeping rituals that had to be done in advance as well. A particular goat or lamb had to be chosen, leavened items were to be removed from the home, etc. Moses gave all these instructions to the elders of Israel to help them prepare.