Le 23:5 In the fourteenth day of the first month at even is the LORD’s passover.
This Feast is what the Bible refers to as The Lord’s Supper, with all of its rich symbolism. In Exodus 12:8 God specifies specific symbolic components of the Passover meal: roasted lamb, bitter herbs and unleavened bread. We understand from the Gospel accounts that one particular instance of unleavened bread, the affikomen, represents the broken body of our Lord (Mat 26:26) and that a particular cup of wine, the cup of blessing (or redemption) (1Co 10:16), which is also a traditional part of the meal, symbolizes the blood of the New Covenant. (Mat 26:28) The sacrificial Passover lamb is also supremely representative of Christ. (1Co 5:7-8)
Evidently due to the intense symbolism in this meal and the overall meaning and significance of Passover in God’s economy, there is a special requirement of personal holiness in those who participate, symbolized in the requirement of circumcision for any male participants and the use of unleavened bread during the meal itself. Passover is the only feast like this, which requires personal holiness in order to celebrate, and Paul indicates that it is possible for a believer to actually get physically ill or even die if they participate in this feast with open, willful, unconfessed sin in their life and consume the symbolic elements of the meal in an unworthy manner. (1Co 11:27-32)
Another unique aspect of this feast lies in the command to recount the glories of the Exodus to all succeeding generations. (Ex 12:24-27) Many reason that this implies the feast is uniquely for Jews, not for Gentiles, claiming Gentiles weren’t delivered out of Egypt so it makes no sense for them to remember it. However, Peter and Paul both note that the people of God are of no single nationality, but are the people of faith. (1Pe 2:10, Ro 9:8, Ga 3:7) When we believe on Christ we become one with His people, the people He delivered from Egypt, as if we were there ourselves.
He wants all believers to remember this day, the day He brought us forth out of the land of Egypt, all the days of our lives. (De 16:3) As we do this we’re naturally reminded of the vast power God displayed in saving us from our captivity to the god of this world and freeing us to follow Him. Setting captives free from such a powerful enemy is no small thing, and the fact that He has already displayed such awesome power in our lives should encourage us whenever we face daunting circumstances. Nothing is impossible with God!
It appears that other aspects of the very first Passover, such as sprinkling blood on the lintel and door posts (Ex 12:7) and eating the meal fully clothed, staff in hand, as if prepared at any moment to leave home for good (Ex 12:11), may not have been part of the regular annual Passover celebration since  God did not allow His people to celebrate this feast at home after they had settled in the Promised Land (De 16:5-6a) and so they may have been in tents without access to durable housing in which to celebrate it, and  Jesus did not appear to be eating in any hurry during His last Passover meal with the disciples since John was reclining next to Him during the meal (Jn 13:23), which appears to be a posture of rest and relaxation inconsistent with the spirit of haste in the initial Passover.
The killing of the Passover lamb occurs toward the end of the 14th day as the evening approaches between the 14th and 15th day. (Nu 33:3) This places the actual Passover meal in the evening of the 15th day of the month, so it is the first meal of the next feast, Unleavened Bread, and is therefore eaten on a special sabbath day. However, it was evidently a custom for the Passover sacrifices to be slain over a period of several days leading up to the 14th, so that it was customary for some to eat it on the evening of the 14th, which is evidently when the Last Supper occurred.