When were the Israelites to bring the firstfruits to the priest?
The Israelites were to bring the firstfruits to the priest when they entered the land and reaped the harvest. Specifically, they were to go into the barley field while it was still green, pluck the early ripened heads, and bring a sheaf to the priest as a wave offering.
Les explains that the firstfruits were brought at the time of the barley harvest in the spring. The Israelites were to enter the barley field while it was still green and find the scattered yellow heads that had ripened before the rest of the grain. They were to pluck those early ripened stems with the heads, gather enough for a sheaf, and take it to the priest as a wave offering. This was commanded for when they came into the land and reaped the harvest, as Leviticus says: "When you are come into the land, and ye shall reap the harvest, you shall bring the firstfruits to the priest."