It's all a matter of opinion.
- God Bless
Young's Literal Translation: [Leviticus 23:5-11] 4 These are appointed seasons of Jehovah, holy convocations, which ye proclaim in their appointed seasons: 5 in the first month, on the fourteenth of the month, between the evenings, is the passover to Jehovah; 6 and on the fifteenth day of this month is the feast of unleavened things to Jehovah; seven days unleavened things ye do eat; 7 on the first day ye have a holy convocation (the first annual Sabbath of seven to follow), ye do no servile work; 8 and ye have brought near a fire-offering to Jehovah seven days; in the seventh day is a holy convocation; ye do no servile work.' 9 And Jehovah speaketh unto Moses, saying, 10 'Speak unto the sons of Israel, and thou hast said unto them, When ye come in unto the land which I am giving to you, and have reaped its harvest, and have brought in the sheaf, the beginning of your harvest unto the priest, 11 then he hath waved the sheaf before Jehovah for your acceptance; on the morrow of the sabbath (the 16th) doth the priest wave it.
The reason we know this is a Sabbath (the 15th) is the fact that the Apostles called it one: [John 19:31] The Jews therefore, because it was the preparation (the 14th), that the bodies should not remain upon the cross on the sabbath day (the 15th), (for that sabbath day was an high day,) besought Pilate that their legs might be broken, and that they might be taken away.
Now, the weekly Sabbath was mentioned in verse three and the context beginning in verse five jumps to God's appointed seasons and holy convocations. No one can argue then.....the plain context of scripture is referring to the Sabbath of Unleavened Bread as verses 8-11 flow directly from the establishment of that high Sabbath that the Apostle John mentions as the day after preparation.
As "Pmary" has mentioned....the beginning of the Count of the Omer according to the Pharisees always began on the 16th....the day after the First Sabbath of Unleavened. Pentecost was known as the Feast of Weeks (49 days from Pesach to Shavuot/from leaving Egypt to the giving of the law at Sinai). It was not known as the "Feast of Weeks, and a couple of days here and there". Consequently, it always falls on Sivan 6 as the Sabbath in question [John 19:31] always falls on Nisan 15.
The reason we know to follow the Pharisees....and not the Sadducees is based upon what Our Lord taught the disciples in [Matthew 23:1-3] 1 Then spake Jesus to the multitude, and to his disciples, 2 Saying The scribes and the Pharisees sit in Moses' seat: 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. He called them hypocrites (they were) but He said they taught the correct observances. This is New Testament teaching directly from Our Lord, Himself!
Josephus ran a close second to the Apostle Paul as the most famous Pharisee of the first century. Here is what Josephus says about the dating of "Passover, Unleavened and First Fruits":
[Antiquities III, Chapter X, paragraph 5] In the month of Xanthicus , which is by us called Nisan, and is the beginning of our year, on the fourteenth day of the lunar month, when the sun is in Aries, (for in this month it was that we were delivered from bondage under the Egyptians,) the law ordained that we should every year slay that sacrifice which I before told you we slew when we came out of Egypt, and which was called the Passover; and so we do celebrate this passover in companies, leaving nothing of what we sacrifice till the day following. The feast of unleavened bread succeeds that of the passover, and falls on the fifteenth day of the month, and continues seven days, wherein they feed on unleavened bread; on every one of which days two bulls are killed, and one ram, and seven lambs. Now these lambs are entirely burnt, besides the kid of the goats which is added to all the rest, for sins; for it is intended as a feast for the priest on every one of those days. But on the second day of unleavened bread, which is the sixteenth day of the month, they first partake of the fruits of the earth, for before that day they do not touch them. And while they suppose it proper to honor God, from whom they obtain this plentiful provision, in the first place, they offer the first-fruits of their barley, and that in the manner following: They take a handful of the ears, and dry them, then beat them small, and purge the barley from the bran; they then bring one tenth deal to the altar, to God; and, casting one handful of it upon the fire, they leave the rest for the use of the priest. And after this it is that they may publicly or privately reap their harvest. They also at this participation of the first-fruits of the earth , sacrifice a lamb, as a burnt-offering to God.
As you can readily see.....the Pharisee method of determining Pentecost will count exactly 50 days between Passover/Pesach and Shavuot/Pentecost. This is why the holiday will not occur on the same day of the week from year to year. It will always occur on the same date (Sivan 6).....however as all of God's Annual Feast days and Sabbaths do.
Now, I personally do not know what your particular faith or confession is.....but I can tell you this: The First Century Christian Church of the Apostles continued to celebrate all of God's Holidays on the correct days. Here are disciples of the Apostle John .....late in the second century observing the Holy Sabbaths as God intended....and as the Apostle had instructed. This is late second century before the Church stepped in and altered various celebrations in an attempt to rid itself of anything that could be construed as "Too Jewish"!
Just ‘Search’ it at Google or visit the ‘ Christian Classics Ethereal Library ’ online (an official site) where you can see for yourself the (~367) canon laws such as; #16, #29, #37, #38, #49. There you will see how the early church in its sensitivity began to initiate its own identity amongst members by slowly segregating them away from any long lasting affiliation to Judeo ties.