Proponents of a Sunday Pentecost have tried to use Joshua 5:10-12 to prove their case. They say, since the Israelites ate of the "produce" of the promised land on Abib 15, the omer or wave sheaf had to have been offered on Abib 14 (Lev.23:14 forbade eating the produce of the land until the sheaf was offered). In their mind, that would make Abib 14 a weekly Sabbath that year; for the omer had to be offered "on the morrow after the [weekly] sabbath."
First of all, a weekly Sabbath on Abib 14 means they are not using the Sabbath "that falls within the Feast of Unleavened Bread". Abib 14 is outside the Feast. Second, their primary premise is wrong. It is assumed that since they ate the "produce" on Abib 15, the sheaf was offered on Abib 14. The truth is, in obedience to Yahweh's command, the Israelites offered the sheaf on Abib 16 but did not eat of the produce until after the sheaf was offered. According to the Mishnah, the sheaf was reaped just after the high sabbath ended and offered before 9:00 am. The Israelites were permitted to eat the produce anytime after the sheaf was offered. Therefore, they ate after approximately 9:00 am on Abib 16. That is when they "did eat of the fruit [produce - Heb. tebuwah] of Canaan" (verse 12). Once the manna had ceased on Abib 16 and the omer was offered, they ate from theproduce of the land.
If that is true, then what was it they ate on Abib 15? Josh 5:11 says they ate "old corn," "unleavened cakes," and "parched corn ." The word "corn" in "parched corn" appears in italics in the KJV which means it is not found in the Hebrew. "Unleavened cakes" is a translation of the Hebrew word "matstsah." "Old corn" is a translation of the Hebrew word "abuwr." The only use of abuwr in the scriptures is found in Josh.5:11,12. It is #5669 in Strong's Concordance and means, "passed, ie. kept over; used only of stored grain." Abuwr comes from "abar" meaning, "to cross over;" It is also translated as, "carry over, bring, pass over, send over." It is obvious that "old corn" refers to the "victuals" that Joshua commanded the Israelites to prepare for their journey across the Jordan in Jos.1:10-11. "Then Joshua commanded the officers of the people, saying, Pass through the host, and command the people, saying, Prepare you victuals ; for within three days ye shall pass over this Jordan, to go in to possess the land, which Yahweh your Elohim giveth you to possess it." The Israelites were not eating any produce from the promised land on Abib 15. They were eating provisions that were carried over the Jordan from land that was not part of their inheritance. Eventually two and one half tribes settled on the east side of Jordan where the old corn came from, but that wasn't the original land of inheritance.
In Lev.23:10-11, Yahweh makes it clear that the wave sheaf or omer must be from the harvest of "the land which I give unto you." The food that was prohibited in Lev.23:14 were foods from the harvest of the promised land. They could not eat "bread, nor parched corn, nor green ears" made from that harvest until they offered the omer. Josh.5:10-12 makes no mention of green ears [karmel], bread [lechem], or parched corn [kahlee]. It only mentions old corn [abuwr], and unleavened cakes [matstsah] that were parched.
One more point about Joshua 5:12. It states the manna ceased the day after they ate the old corn (or food that was carried over the Jordan). The miracle of the manna in Exodus 16 taught the Israelites that manna would always cease on the weekly Sabbath. Sunday Pentecost keepers claim that Abib 14 was a weekly Sabbath that year. That would mean the manna ceased on a Monday. If Yahweh is consistent, the manna of Josh.5:12 would have again ceased on the weekly Sabbath.