When did Israel celebrate the Feast of First Fruits (commanded in Leviticus 23:9 - 14)?
Today, in my reading of Joshua chapter 5, I found an indication of the day when the people of Israel ate the "first-fruits" of the land at Gilgal, after 40 years of eating manna, just after they crossed the River Jordan into the Promised Land. That was the day after the Passover, the 15th day of the First Month, the first day of Unleavened Bread. On that day, the supply of manna ceased.
11 The day after the Passover, that very day, they ate some of the produce of the land: unleavened bread and roasted grain. 12 The manna stopped the day after they ate this food from the land; there was no longer any manna for the Israelites, but that year they ate the produce of Canaan.
Joshua 5:11 - 12 New International Version
I made a similar observation about one and a half years ago, in my devotional blog post: