"For I have received of the Lord which also I delivered unto you, that the Lord Jesus the same night in which He was betrayed took bread: and when He had given thanks, He brake it, and said, 'Take, eat: this is My body, which is broken for you: this do in remembrance of Me.' After the same manner also He took the cup, when He had supped, saying, 'This cup is the new testament in My blood: this do ye, as oft as ye drink it, in remembrance of Me.' For as often as ye eat this bread, and drink this cup, ye do shew the Lord's death till he come" (1 Cor. 11:23-26).
The occasion for the first "Lord's Supper," was, I believe, at least twenty-four hours before the time the Passover was to be killed (Jn. 13:1). Since Jesus is declared to be the Passover Lamb (Jn. 1:29, 36; 1 Cor. 5:7; 1 Pet. 1:19), I believe He died exactly when the Passover lambs were to be slain (Ex. 12:1-28; Num. 9:1-5; 28:16-25; Lev. 23:4-5). He was on the cross on the "day of preparation" for a special Sabbath (Jn. 19:14, 31). That "special Sabbath," I believe, was the Passover.
There are two symbolic meanings of leaven in the New Testament: there is the yeast used to make both bread and wine, always representing sin (Mt. 13:33; Lk. 13:21; 1 Cor. 5:6-8; Gal. 5:9), and there is the use of "leaven" as a metaphor for false teaching (Mt. 16:6, 11-12; Mk. 8:15; Lk. 12:1). The "Lord's Supper" consisted of unleavened bread, and unfermented wine, because only they could represent Christ's sinless body (2 Cor. 5:21; Heb. 4:15; 9:28). As for the false teachings of the Pharisees and the Sadducees, they rejected Jesus. Basically, their view of Him was the exact opposite of the truth.
The most important thing about believers partaking in the "Lord's Supper,"
is that in so doing, we remember what Jesus did at His First Coming,
and that there will be a Second Coming! Praise God for both!