Pesah is the 14th day of the first month of the Biblical year- the month of the Aviv. This begins at evening, at the close of the 14th day. This sacred day commemorates the night in which YHWH sent the destroyer through Egypt and killed the first-born of Egypt- resulting in the release of the Israelites from Egyptian servitude.
Hag ha-Matzoth, the Festival of Unleavened Bread, is the 15th day of the first month of the Biblical year. Hag ha-Matzoth commemorates the day on which the Israelites left Egypt and began their journey toward mount Sinai to receive the Torah.