Passover
Passover to day starts the Jewish religious calendar. It is celebrated on the 14th day of the month of Nisan which is either in the month of March or April.
Passover refers back to when God said to Moses about the last of the Ten Plagues. God sends a angel of death who would kill the first born of any one who did not have lamb's blood on the door of their house.
Passover was the day the Hebrews left Egypt.
Background Reading:
The Passover - Numbers 28:16-31
16 "On the fourteenth day of the first month the LORD's Passover is to be held. 17 On the fifteenth day of this month there is to be a festival; for seven days eat bread made without yeast.18On the first day hold a sacred assembly and do no regular work.19Present to the LORD an offering made by fire, a burnt offering of two young bulls, one ram and seven male lambs a year old, all without defect.20 With each bull prepare a grain offering of three-tenths of an ephah of fine flour mixed with oil; with the ram, two-tenths;21and with each of the seven lambs, one-tenth. 22 Include one male goat as a sin offering to make atonement for you. 23 Prepare these in addition to the regular morning burnt offering. 24 In this way prepare the food for the offering made by fire every day for seven days as an aroma pleasing to the LORD; it is to be prepared in addition to the regular burnt offering and its drink offering. 25 On the seventh day hold a sacred assembly and do no regular work."
Also read Exodus 12:1-51, Leviticus 23:4-14 and Numbers 9:1-14