I believe there is genuine/sincere confusion in the "watching for Jesus" group of folks concerning how the Feast of Shavuot relates to the Feast of Pentecost.
Shavuot is at the point of 7 complete Sabbaths counted from the Passover's Feast of Firstfruits. Pentecost takes place 50 days after that. Shavuot takes place at about the middle of the 3rd month. Pentecost takes place at about the beginning of the 5th month.
Some believe that Shavuot is a high watch time since, in their thinking, that is when the two loaves of leavened bread of wheat were to be raised before God by the High Priest, thus symbolizing a rapture type/shadow. They think that this is the second of three Feasts that the men of Israel were to gather before God in Jerusalem.
Indeed, Shavuot is the timing of a wheat harvest. It is the wheat that is planted in the late fall, and takes about 7 months to grow. It is ready for harvest May-ish. See Deuteronomy 9:22-32 and especially verse 32. The plague of hail (sometime before the 1st month) did not kill the winter wheat because it had not developed much above ground.
However, Pentecost is also the timing of another, different type of wheat. It is planted in the spring and takes about 4 months to grow. It is ready for harvest July/August. The kernel is not as dense/hard as the winter wheat grain and so it made for a lighter bread.
God wanted first fruits of each and every grain harvest. So, the offering of both types of wheat was expected/commanded.
The question is: Which of the two wheat harvests did God want its first fruits offering to be presented in the form of two leavened loaves of bread?
A big clue is found in Exodus 23:16:
Also you shall keep the Feast of the Harvest of the first fruits of your labors from what you sow in the field; also the Feast of the Ingathering at the end of the year when you gather in the fruit of your labors from the field.
For the Feast of the Harvest, note "from what you sow in the field." Lets apply this to the first year the children of Israel entered the Promised Land.
Joshua and the fighting men of the tribes were beginning about a 7 year long campaign where they were constantly fighting and removing the inhabitants. They were not in farming mode.
However, the families of Gad, Reuben and the half tribe of Manassah were settling into their land allotments on the other side of the Jordan. The fighting men ages 20 and up (see the age validation in Numbers 1:2-3 with 26:2-4) stayed with Joshua and the other tribes to fulfill their prior promise to Moses to do so. Those three tribes' families were beginning the life of farming at the beginning of the year and so also were going to fulfill God's instructions concerning the first fruits. Those first fruits would be presented to God at wherever the Tabernacle had been erected by the Levites. Those families had a lot of young teenage "men" to help do the hard work of farming. Those young men (and women) had been born in the wilderness. They were fit due to all of the wanderings. They were in excellent health because they had been existing solely off of God's manna, a complete, nutrient dense food. They were all in top shape.
So, those families of Gad, Reuben and Manassah would also fulfill the requirement for the first fruits offering of the wheat harvest by planting seed right away. The barley was already getting ready to harvest when they settled in their promised land. The winter wheat was also already growing, approaching its harvest window. Could they offer the two loaves of wheat from that winter wheat harvest? No, because they had not planted it. The loaves were going to be made from the wheat they were going to plant that spring. That wheat would start to be ready about four months later, and in time for the Feast of Pentecost (the Feast of the Harvest referenced in Exodus 23:16 above). The four months til the harvest window started would end on about the end of the 4th month to the first part of the 5th month.
The two loaves of wheat could NOT have been made from the winter wheat during that first year. It had to be from what they themselves planted.
[It is my guess that the families of the each of the fighting tribes may have all lived in the land of Gad, Reuben and Manassah, at least during some of the initial phases of the fighting campaign. Also, the three tribes themselves had a lot of animals (Numbers 32:1). Basically, the families living in those areas would supply the priests and the fighting men with food, especially meat, above and beyond what those men could glean off of the land as they went along. No longer eating manna, those men needed a high meat diet.]
I believe Amos 8 shows the timing of Pentecost verses Shavuot because they have to wait for both the Sabbath to be done AND also the New Moon. The ending of the 7 Sabbath count for Shavuot does not end at the New Moon timing. The greedy, impatient sellers wanted to start selling wheat from a recent harvest (they wanted to open the market) but could not until the first fruits was offered.
Amos 8 also points to a Pentecost season timing for when God will bring a judgment against Israel (this year?) including what seems to be not only a massive earthquake but also a rotational "shifting" of the earth crust (similar to Hezekiah's time?). Some of the language suggests that a major transition has taken place in terms of how God was going to relate to Israel because, at some point, revelation from God will not be readily available. This is part of the judgment. Recall Peter's sermon in the first part of Acts as he quotes from Joel at the timing of Pentecost. He points to pillars of smoke. In Joel, that word "pillars"' meaning includes palms, as in palm trees. Pillars of smoke from volcanos or large bombs? If we go on a Pentecost, it will be back to back with severe judgment unfolding on the earth, lending significance to the word harpatzo.
"1This is what the Lord [a]GOD showed me, and behold , there was a basket of summer fruit . 2And He said , “What do you see , Amos ?” And I said , “A basket of summer fruit .” Then the LORD said to me, “The end has come for My people Israel . I will not [b]spare them any longer . 3[c]The songs of the palace will turn to wailing on that day ,” declares the Lord GOD . “The corpses will be many ; in every place [d]they will throw them out . [e]Hush !”4Hear this , you who [f]trample the needy , to put an end to the humble of the land , 5saying ,
“When will the new moon [g]be over ,
So that we may sell grain ;
And the Sabbath , so that we may open the wheat market,
To make the [h]ephah smaller and the [i]shekel bigger ,
And to cheat with dishonest scales ,
6So as to buy the helpless for [j]money ,
And the needy for a pair of sandals ,
And that we may sell the refuse of the wheat ?”
7The LORD has sworn by the pride of Jacob ,
“Indeed , I will never forget any of their deeds .
8“Because of this will the land not quake ,
And everyone who lives in it mourn ?
Indeed, all of it will rise up like the Nile ,
And it will be tossed about
And subside like the Nile of Egypt .
9“And it will come about on that day ,” declares the Lord GOD ,
“That I will make the sun go down at noon ,
And make the earth dark in [k]broad daylight .
10“Then I will turn your festivals into mourning ,
And all your songs into songs of mourning ;
And I will put sackcloth around everyone’s waist ,
And [l]baldness on every head .
And I will make it like a time of mourning for an only son ,
And the end of it will be like a bitter day .
11“Behold , days are coming ,” declares the Lord GOD ,
“When I will send a famine on the land ,
Not a famine of bread or a thirst for water ,
But rather for hearing the words of the LORD .
12“People will stagger from sea to sea
And from the north even to the east ;
They will roam about to seek the word of the LORD ,
But they will not find it.
13“On that day the beautiful virgins
And the young men will faint from thirst .
14“As for those who swear by the [m]guilt of Samaria ,
And say , ‘As your god lives , Dan ,’
And, ‘As the way of Beersheba lives ,’
They will fall and not rise again .”