Paul takes us all the way back to Leviticus 23 and the inauguration of the Feast of Firstfruits, where the very first sheaf of grain in every smallholding, every farm in ancient times, was to be brought to the priest and the priest would wave it in the air. This was before the main harvest; the rest of the harvest was brought in later, and the end of harvest was marked by the Feast of Weeks seven weeks later (Leviticus 23:15-22).
Sleep does not refer to the soul, but to the body. All those people who believe in Christ, who love him and trust in him; all those who belong to God, when they die, their bodies, of course, perish and rot; but their bodies are said to sleep. They are not asleep, literally; of course not. They are dead, as the apostle goes on to affirm shortly. Nevertheless, sleep is the appropriate term, because one day they will be raised again. Christ used the same figure in describing Lazarus: ‘Our friend Lazarus sleepeth; but I go, that I may awake him out of sleep. Then said his disciples, Lord, if he sleep, he shall do well. Howbeit Jesus spake of his death’ (John 11:11-13). But it doesn’t refer to the soul, because at death the soul of the believer goes to the paradise of Christ, awaiting that coming final day of resurrection.