It is a standard of Christian teaching that Christ was raised on Firstfruits, but there is some detail within the harvest law of Firstfruits that does not seem to be well understood or explained in today's Christian programing.
In 1Cor 15:20 Paul states "But now is Christ risen from the dead, and become the firstfruits of them that slept." it is quite natural in our thinking to interpret this as Christ was raised "as the Firstfruits" this verse is rather difficult to accept as it reads in the KJV among some of the Christians with a broad understanding of the Leviticle Law, some prefer to omit "and become" and some will interpret this as "first of the firstfruits" but in any case, as it reads in the KJV the words "and become" are clear, this word in the "received text" textus receptus is "Ginomai" strong's number 1096 and was interpreted into the KJV as "and become" this very same word "Ginomai" Has a wide range of interpretation being a verb of the middle voice in Greek , and with our English not having a middle voice the interpreters are left up to following the context of what Paul is writing.
As to the definition of the firstfruits according to God, the firstfruit loaves are made with a sinful nature, they are prepared and baked with leaven, lev23:17 as they represent God's firstfruits of his harvest Rev 14:4 they are men of Israel. So it would not be possible for Christ himself to become the Firstfruits by God's own definition. But as Paul teaching the Resurrection of Christ in 1Cor 15, Christ is the Resurrection, and so "become" the firstfruits of them that slept, because the firstfruits require a resurrection to be the firstfruits to God, THE FIRSTFRUIT IS A RESURRECTION / HARVEST EVENT. Thus Christ has risen from the dead and become the firstfruits/resurrection of them that slept.(As an example), if this verse was to read like this- "But now is Christ risen from the dead, and become the in-gathering for all who believe at the end of the age" it would be clear that Paul is explaining that Christ has become the Resurrection for the feast of in-gathering, but he uses firstfruits and confirms “those that slept” as the firstfruits in doing so.
In Lev 23:10-11 God tells the children of Israel when they begin to reap their harvest they are to bring a sheaf of the firstfruits of the harvest to the priest and the priest shall wave this before the Lord to be accepted for you. What this meant to Israel was that before they could put a sickle to the harvest or even to eat or use any of the harvest they had to bring a sheaf “actually an omer by weight about equal to 2 ¼ quarts of ready to harvest barley to the priest and the priest would lift this omer of barley up to the north the south the east and the west and then after this could the harvest begin.
Now, we know that Christ was raised in the early morning hours on the very day that the priest would lift up the omer to God "wave the sheaf before the Lord" and according to the gospel when Mary saw Jesus the first thing spoken by the Lord was "do not touche me" for I have not yet ascended to the Father, he was obviously in the state of purification as touching the requirement of the High Priest before entering the presence of God, and as the earthly high Priest was preparing to wave the omer before an empty temple, Christ our High Priest forever after the order of Melchizedek was preparing to present the true sheaf before God, the body of his sacrifice so the true Harvest could begin and be accepted of God.
Now, according to Law, there had to pass seven Sabbaths upon the fiftieth day from the day Christ was raised could the firstfruit loaves be waved before God, and thus begin the festival of weeks, or Pentecost (which means fifty) now there is no specific time of reference in Matthews writing of when the graves were opened and many body's of the saints that slept arose, only to say after his resurrection, but as touching the Law, this could only take place on the day of the waving of the firstfruit loaves "Pentecost", while the Apostles had received the Holy Spirit on Pentecost, it would be certain that God received his firstfruits from the first covenant. For Christ himself said that the scriptures cannot be broken, and that he came to fulfill the Law. So Christ was raised in what is referred to in Jewish tradition as "Reshit Katzir" meaning "beginning of the harvest" and fifty days following was the firstfruit loaves waved before God according to Law.
Christ was raised as our High Priest forever to offer up to God the Body of his sacrafice of which it is written that he was perfected in the suffering for this very cause, it is only after this God would receive and accept the Firstfruits of his Harvest