What was the meal that the Lord ate with His disciples on Covenant Thursday? If it were Passover, why were they sitting at the table and not standing according to the Holy Book of Exodus 12?
The meal our Lord Jesus ate with His disciples was indeed the Passover after which He instituted the sacrament of the Eucharist at the Last Supper.
The Passover was celebrated on the 14th of the first month (the month of Nissan) with the service beginning in the evening (Lev. 23:6). It was on the evening of this day that Israel left Egypt. Observances connected with the Passover are in two categories, those established at the keeping of the first Passover and those enacted after the Exodus.
The regulations for the first Passover in the land of Egypt are detailed in Holy Book of Exodus 12. However several supplementary regulations were instructed by God for the observance of Passover after the Exodus: all male members of the congregation were to appear before the Lord with "the choice first fruits" (Ex 23:14-19), the first sheaf of the harvest to be offered on "the day after the Sabbath" (Lev 23:10-14) those prevented from keeping the Passover on the 14th Nissan were to observe it on the fourteenth of the following month (Num 9:6-14); special sacrifices were to be offered each day of the festival (28:16-25); Passover was to be observed "in the place which the Lord your God will choose." This implied the sanctuary of the tabernacle or the Temple in Jerusalem; so the paschal animals were to be slain in the national sanctuary and the blood sprinkled on the altar instead of the doorposts and lintels of the homes (Deut 16:1-8).
In New Testament times, Passover became a pilgrim festival. Large numbers gathered in Jerusalem to observe this annual celebration. Christ and His disciples ate the Passover meal together on the eve of His death. Like the blood of the lamb which saved the Hebrew people from destruction in Egypt, His blood, as the ultimate Passover sacrifice, has redeemed us from the power of sin and death.