This question can not be answered with out some important sub-questions asked first, such as: What calendar does one use? Is there more than one calendar that is authorized by God to set a time for any of the Holy Days, not just Pentecost? Who is authorized to set (proclaim) when God's Holy Days are to be kept on an annual cycle?

To answer your question and all the related sub-questions, one must make some important observations. The fact that there are so many "calendars" in use and therefore so many different dates proclaimed by some one, what one sees is an obvious mass confusion on this issue. A confusion that God never intended. 1 Cor. 14:33 "For God is not the author of confusion, but of peace, as in all churches of the saints." Where did this mass confusion arise from? The short and strait answer to that is-from Satan! However, as concerns the Church of God, one must look at as to where it started of relatively recent history, about the last 50 to near 100 years. More about that later.

We must attempt to answer some of the sub-questions asked earlier. Is there more than one God authorized Holy Day calendar for His Church? The short and straight answer to that is-how could there be? That would make God the author of confusion! Next question: Who has the God given authority to Proclaim when the Holy Days are to be kept? Would any baptized member be authorized by God to make such a proclamation? When God set on a course to call a nation of king-priests to minister under Him, He commanded (authorized) Moses to make that proclamation. Lev. 23:1 And the Lord spake unto Moses, saying, (v 2) Speak unto the children of Israel, and say unto them, Concerning the feasts of the Lord, which ye shall proclaim to be holy convocations, even these are my feasts. (emphases mine through out)

Notice the instruction was to Moses, to give to His nation. Now God knew that Moses would not live to the second coming of Jesus to do the proclaiming on an annual basis, so God set up an order or method if you will, where by this would be done. Exd. 24:1 "And he said unto Moses, Come up unto the Lord, thou, and Aaron, Nadab, and Abihu, and seventy of the elders of Israel; and worship ye afar off." Once Moses died, the continuation of the system of seventy elders (known as the Sanhedrin in the time of Jesus Christ) remained until the destruction of the temple in Jerusalem in AD 70 and continued in exile ending in Tiberias several hundred years later. While Moses was alive he and the seventy elders alone had the authority to proclaim when on an annual basis the Feasts of God were to be kept. Moses was of the tribe of Levi and they were God's priest hood to the other eleven tribes of Israel. Exd. 4:14 And the anger of the Lord was kindled against Moses, and he said, Is not Aaron the Levite thy brother...? Exd. 28:1 And take thou unto thee Aaron thy brother, and his sons with him, from among the children of Israel, that he may minister unto me in the priest's office, even Aaron, Nadab and Abihu, Eleazar and Ithamar, Aaron's sons.

It must be noted here that the tribe of Levi had not been given a territory separate of their own as the other twelve tribes, so they dwelt among all the tribes including the tribe of Judah (the law givers) Num. 18:24 But the tithes of the children of Israel, which they offer as an heave offering unto the Lord, I have given to the Levites to inherit: therefore I have said unto them, Among the children of Israel they shall have no inheritance. Num. 35:2 Command the children of Israel, that they give unto the Levites of the inheritance of their possession cities to dwell in; and ye shall give also unto the Levites suburbs for the cities round about them. Deut. 14:27 And the Levite that is within thy gates; thou shalt not forsake him; for he hath no part nor inheritance with thee. Deut. 18:1 The priests the Levites, and all the tribe of Levi, shall have no part nor inheritance with Israel: they shall eat the offerings of the Lord made by fire, and his inheritance. (v 2) Therefore shall they have no inheritance among their brethren: the Lord is their inheritance, as he hath said unto them. Josh. 13:14 Only unto the tribe of Levi he gave none inheritance; the sacrifices of the Lord God of Israel made by fire are their inheritance, as he said unto them.

Next question: Did God give them a calendar where by Moses and the seventy elders could determine what days to make a proclamation for His Feasts to be observed on an annual basis? Had too, or the whole nation would have been in the dark as to what days they were to keep them. Now God would not expect His nation to keep the law of His Feasts with out a calendar, would He? Then who's and what calendar does one use today to be in line with keeping the Law concerning the Feasts of God? Would He have many different calendars or just one, and if just one, which one?

When one looks at the Churches of God recent history, many calendars have sprung up and new ones almost on a daily basis spring up. This can not be of God, not withstanding, all those who have made their own calendar and proclaim with out authorization from God to do so.

Scripture clearly reveals that the tribe of Judah was given to be the law giver Gen. 49:10 'The sceptre shall not depart from Judah, nor a lawgiver from between his feet, until Shiloh come; and unto him shall the gathering of the people be." The phrase "until Shiloh come", refers to the Messiah's (Jesus) second coming. This prophecy of the "gathering of the people" has not taken place yet because Jesus has not yet returned the second time. The Levites taught the Law of God to the tribe of Judah as well as the other tribes, but the tribe of Judah as scripture plainly reveals was the tribe chosen to preserve the Law for all succeeding generations.

They alone have the authority to proclaim the Holy Day dates of God's Feasts. All other "Holy Day calendars" are unauthorized man made calendars. Note also: Psalms 60:7 Gilead is mine, and Manasseh is mine; Ephraim also is the strength of mine head; Judah is my lawgiver; Rom. 3:1 What advantage then hath the Jew? or what profit is there of circumcision? (v 2) Much every way: chiefly, because that unto them were committed the oracles (Strong's Ref. #3051: Romanized logion, Pronounced log'-ee-on; neuter of GSN3052; an utterance (of God): KJV--oracle.) of God.

Next question: Which sect of the Jews are given that authority? The answer is found in Mat. 23:1-3. "Then spake Jesus to the multitude, and to his disciples, (v 2) Saying,The scribes and the Pharisees sit in Moses' seat: (v 3) All therefore whatsoever they bid you observe, that observe and do; but do not ye after their works: for they say, and do not." [It should be noted here that scribes were Levites] II Chr. 34:13 Also they were over the bearers of burdens, and were overseers of all that wrought the work in any manner of service: and of the Levites there were scribes, and officers, and porters. No where is that said of the sect of the Jews called Sadducees and they kept a Sunday Pentecost as do the Karaite Jews today who trace their origins back to the Sadducees. The Pharisees in Jesus time kept a Sivan six Pentecost, which on the Hebrew Calendar can be on other days of the week besides Sunday or Monday.

Many will argue this point and the objections are long and wide. One thing to remember though, there is not much confusion among the Jewish people what Days they proclaim as Holy, as God's Feasts.

With that point in mind, we come back to the question, why all this confusion in the Churches of God? The Holy Days were taught by, Elder, Armstrong to be kept. As a highly influential leader, his "authority" held great weight to the timing of the keeping of God's Feasts by most people under his tutorship. He did not adhere to the Hebrew Calendar in it's entirety, mainly that of Passover and Pentecost. Therefore, that was and is to this day, the main cause of the many calendar claims and the resultant mass confusion on this issue. Had, Elder, Armstrong, with his great influence, not "proclaimed" another calendar, one would see very little disagreement on this issue today. Had he recognized the Hebrew Calendar as kept by most Jews through out the world today, most of his followers would surely be keeping the Hebrew Calendar in it's entirety as well, including Passover-Unleavened Bread 15th observance and Sivan six Pentecost.

One may want to argue that point as well, but the record should speak for itself. [Mass confusion eliminated before it ever got started.] And if there still were questions as to the validity of the Hebrew Calendar, in spite of the Scriptures quoted above we are assured all things will be corrected when the King of Kings returns. Acts 3:20 And he shall send Jesus Christ, which before was preached unto you: (v. 21) Whom the heaven must receive until the times of restitution of all things, which God hath spoken by the mouth of all his holy prophets since the world began.

The original question has been answered. Which Pentecost day do you want to observe; Sunday, Monday or Sivan six?