The start of this harvest-period is intrinsically linked to the end of it. One event marks the start, and another event marks the end, with a 50-day count inbetween both. The word 'feast' is used, and the time of year this took place was the Jewish first month of Abib, which corresponds to our March and April. The firstfruit was when a few stalks of grain ripened, enough to make up one sheaf. They were the harbinger of the full harvest when the main crop would have ripened, 50 days later. This one sheaf was presented at the temple, dedicated to God. The priest had to wave the sheaf before the Lord, and a burnt offering of a perfect lamb one year old made, along with its grain and wine offerings also (Leviticus 23:9-14).
The count in Leviticus 23:15-21 (known as the Feast of Weeks) started from the day after the Sabbath, the day that first sheaf of ripened grain had been waved before the Lord. Seven full weeks equaled 50 days, and was known as Pentecost. That was when all the Israelites were to then bring their own grain, lamb and drink offerings to the temple. Obviously, this was a massive gathering and a celebration, a feast.
The first, single sheaf prefigured Christ as the first-fruit, ascending to heaven, then 50 days later came Pentecost when the harvest following began on earth. On that one day alone, 3,000 converts to Christ were 'brought in' (harvested). This is where a look at the word applied to Jesus - prototokos - is revealing. As per the website link given in a comment above, I now quote from it:
"Nine times is the word used in the Greek scriptures. The word does
not mean 'first-begotten' nor does it - merely - convey the idea of a
first bearing. Nor should it be confused, in translation or in
concept, with the word monogenes... in the nine times it is used,
prototokos expresses the first fruit, the first return but in different contexts. The Person who is the Prototokos is seen to be
such in different aspects, seven times. Twice, the word is used in a
wider sense, once pre-figuratively and once in anticipation.
...The Prototokos was brought forth of Mary. He was The Son - of her.
And Joseph, as he was commanded by an angel, called his name Jesus.
This was the first indication of God's investment in creation. All
that labour. All that patience. All that burdensome suffering of the
vessels of wrath fitted for destruction. All the patient bearing with
Israel in order to demonstrate his purposes on earth in artefact and
ritual, before the coming of the Messiah.
All of God's investment, now proved to be a viable project. For here
was the first return of all that input. Here was the 'firstfruit' of
that creation, The Prototokos.
That creation - so lamentable in its failure - had now, at last,
brought forth the first token of success. And his name was called
Jesus, for he shall save his people from their sins." (The Only
Begotten Son of God, Nigel Johnstone, pp24-25, Belmont Publications
I hope this now enables clearer understanding of the significance of Christ, as the firstfruit, as per 1 Corinthians 15:20 - "But now is Christ risen from the dead, and become the firstfruits of them that slept." The firstfuit offering had to be made first, and it guaranteed the main harvest to follow.