The Talmud: Selections, by H. Polano, , at sacred-texts.com
The Feast of Weeks, or "Pentecost," occurs upon the sixth day of the third month, Sivan (June). It is called the Feast of Weeks because forty-nine days, or seven weeks, duly numbered, elapse between the second day of Passover, when (during the existence of the Temple) a sheaf of green barley was offered, and this festival, when two loaves made of the first flour of the wheat harvest were "brought before the Lord." It is also the anniversary of the delivery of the commandment from Mount Sinai.
Why does not the Bible particularise in this as on other occasions, and say directly, "On the sixth day of the third month was the law given?"
Because in ancient times the men called "wise" placed their faith and dependence upon the planets. They divided these into seven, apportioning one to each day of the week. Some nations selected for their greatest god the sun, other nations the moon, and so on, and prayed to them and worshipped them. They knew not that the planets moved and changed according to the course of nature, established by the Most High, a course which He might change according to His will, and into their ignorant ideas many of the Israelites had entered. Therefore, as they considered the planets as seven, God made many other things depending on that number, to show that as He made them, so had He made the planets.
The seventh day of the week He made the Sabbath; the seventh year he made the year of rest; after seven times seven years, or after seven Sabbatical years, He ordained the Jubilee, or year of release. Seven days He gave to the Passover festival, and seven days to the Feast of Tabernacles. Seven days was Jericho surrounded, and seven priests took seven trumpets and marched round its walls seven times upon the seventh day.
Therefore, after numbering seven weeks during the ripening time of the grain, the Israelites were to hold a holy convocation, to praise the One who can prevent all things, but who cannot be prevented; who can change all things, but is unchangeable.
The first day the Israelites were redeemed from slavery and superstition; the fiftieth day a law was given them for their guide through life; therefore they are commanded to number these days and remember them.
The children of Ishmael, says the legend, were asked to accept the law. "What does it contain?" they asked. "Thou shalt not steal," was the answer. "How can we then accept it," they returned, "when thus was our forefather blessed, 'Thy hand shall be against every man?'"
The children of Esau were asked to accept the law, and they also inquired, "What does it contain?" "Thou shalt not kill," was the answer. "We cannot accept it, then," said they, "for thus did our father Isaac bless us, 'By the sword shalt thou live.'"
When Israel was asked to accept the law, the people answered, "We will do and obey."