Exodus 23:18 "You shall not offer the blood of my sacrifice with leavened bread"

Only after your mother has cleaned out the kitchen and stored away all the regular dishes which are chametz, does she unpack the Pesach dishes and start cooking for the holiday.

The Paschal sacrifice must be brought on the afternoon of the day before Pesach. However, before the sacrifice can be brought, all chametz must be removed from our possession.