The current observance of Shavuot has no relationship to its biblical ancestor and doesn’t even occur at the same time. Very few people know the truth about this day. Most Jews think Shavuot recalls the day the Torah was revealed to the Israelites during the days of Moses. This is not true. This significance was given to the holiday in the middle ages when the holiday had lost one of its original purposes; the sacrifice prescribed for the day was discontinued when the second Temple was destroyed in 70 CE. Additionally, the Bible states that only the Decalogue, called the Ten Commandments even though it contains more, was revealed on a specific day. The Bible states that the rest of the Torah was given during different times during the next thirty-eight years. 
The Hebrew name “Shavuot” means “weeks,” and the Latin name “Pentecost” means “fiftieth.” Both refer to the commands in Exodus 34:22; Deuteronomy 16:9, 10, and 16; and Leviticus 23:15 and 16, where the Bible tells the Israelites to count seven full weeks after the Sabbath and states that the fiftieth day is Shavuot, when a prescribed sacrifice was to be brought. What, then, is the purpose of the biblical Shavuot? Why was this special day instituted? Why were the Israelites commanded to count seven weeks of seven days?
The Bible says in Exodus: “You should observe the holiday of Shavuot, as well as the first-fruits of wheat harvest (Passover), and the feast of ingathering at the turn of the year (Sukkot).” This verse does not explain what Shavuot is. Deuteronomy writes: “You should count seven weeks, begin to count from the time the sickle is first put to the standing corn.” Deuteronomy focuses on “weeks,” that the counting should be of seven weeks. It prescribes that a freewill offering should be brought on Shavuot and it should be a time of rejoicing. It does not tell us why the weeks are counted and why the holiday was instituted. Leviticus is more specific: “You should count from the morrow of the Shabbat from the day that you brought the sheaf of the waiving; seven full weeks; count fifty days until the morrow after the seventh Shabbat; then present a new meal offering unto the Lord.”
A controversy arose between the ancient Sadducees, Pharisees, and Christians regarding the twice-mentioned “Shabbat” in the verse and the concept of “seven full weeks.” The Sadducees, a word meaning “righteous ones,” people who upheld the ancient literal understanding of biblical verses, maintained that “Shabbat” is the weekly seventh day. Since the “sheaf of waiving” occurred on the first day of Passover, the count began on the day following the next Shabbat, on Sunday. The counting ended on the morrow after the seventh Shabbat of the counting, again on a Sunday. According to them, this is the plain sense of “seven full weeks,” for a week begins on a Sunday and ends on the Shabbat. This interpretation is consistent with the fact that the Bible requires the counting of weeks, for a week begins on a Sunday. The Sadducees were convinced that Shavuot always fell on a Sunday, but because in ancient times months began when witnesses saw the first sliver of the moon, and this date varied, the exact date of Shavuot is impossible to predict; it could be any of several days in what was called the month of Sivan when the month received this name around 550 BCE, during the Babylonian exile.
The Pharisees, a word that can be defined as “separatists,” disagreed. They insisted that the first mention of Shabbat is the first day of the holiday of Passover. They said that besides denoting the seventh day “Sabbath,” Shabbat means rest, and the first day of Passover is a day of rest. They said that the second appearance of Shabbat, however, has a different sense, “weeks.” They also contended that “seven full weeks” does not signify “Sunday to Saturday,” its literal sense, but seven times seven days.
Why did the Pharisees develop this seemingly forced interpretation of Scripture? I have seen no answer to this question, and suggest the following: They were bothered by the fact that the Bible gives no date for Shavuot and that Shavuot could occur, as previously stated, on one of three possible dates. By starting the count on the day following the first day of Passover, whose date was set by the rabbis as 15 Nissan, and by establishing when the following two months would start, rather than the usual practice of using witnesses for these months, they were able to set the date of Shavuot as 6 Sivan. They then invented the idea that the Torah – and by Torah, they must have intended the Decalogue – was given on this date, even though the Torah itself doesn’t mention the date for the Sinai revelation. This, then, is the secret origin of today’s Shavuot. It is not a biblical observance. It is the taking of a biblical day, moving it to a desirable place, and giving it a totally new significance.
Three questions remain: (1) Why did the Bible institute Shavuot? (2) Why did the Torah mandate that the Israelites count seven weeks of seven days?
Seven is an important number in Judaism. The Bible begins with the story of creation, that God created the world in six days and ceased creation (rested) on the seventh. Jews are instructed to observe the seventh day as Shabbat. Among other things, Shabbat reminds the Jew of God, that God created the world, and God gave people certain laws. To emphasize this important lesson, the number seven reoccurs many times in Jewish holidays and practices. I gave examples of well over a hundred usages of the number seven in Judaism in my book Maimonides and the Biblical Prophets.
Among the others, I pointed out that seven is used for days (the Sabbath), months (the holiday of Rosh Hashanah occurs at the outset of the seventh month), years (the seventh year is the Sabbatical Year), Sabbatical Years (the fiftieth year is the Jubilee Year). What is missing in this constant reminder of the basic teaching is weeks. This was and is the purpose of Shavuot, which is called “weeks” because this is its purpose, the counting of seven weeks recalls the message of seven, that there is a God who created the world, and gave commands.
 I am not advocating that Jews should not observe Shavuot. Judaism today is not Torah Judaism. It is Rabbinic Judaism. Jews observe the Torah as it is interpreted by the rabbis. I observe Shavuot. The purpose of this article, from my book “Mysteries of Judaism,” is simply to reveal the historical development of Shavuot.
 Scholars differ about the count: eleven, twelve, and thirteen commands. The Torah does not use the term Ten Commandments, but Ten Statements, which is the meaning of Decalogue. Some of the statements have more than a single command.
 Actually, the biblical Passover was on 14 Nissan and Chag Hamatzot, the seven day Festival of Matzot, began on 15 Nissan. The only observance of the biblical Passover was the eating of the Pascal sacrifice. When the temple was destroyed in 70 CE and sacrifices stopped, the holiday of Passover ceased to exist. The rabbis solved the problem by starting to call the Festival of Matzot by the name Passover. The siddur, prayer book, continues the ancient practice of calling the festival Chag Hamatzot. The turning of Chag Hamatzot into Passover is another example of today’s Jewish holidays being different than those in the Bible.