The five megilot are each connected to five occurrences in the year. Shir Ha'Shirim
is read on Pesah, Ruth is read on Shabouot, Echa is read on Tishah BeAb, Qoheleth
is read on Succoth and Esther is read on Purim. There are many important reasons
tying Megilat Ruth to Shabouot.
Why is Megilat Ruth connected to Shabouot?
1. Lineage of David Hamelech:
On Shabouot, David Hamelech was born. We read Megilat Ruth as a testimony to prove
David's lineage and that he is the justified, undisputed king of Israel.
How is his lineage shown in the Megilah?
Rashi explains in the verse "Vayasem Kiseh Lamelech" (Melachim Bet 2:19)- that Shelomo
Hamelech placed a seat for the "king's mother", which is meant to refer to Ruth,
who was the mother of the kingship of Israel. She was blessed with many years of
life, living to the time of Shelomo, her great-great grandchild.
2. Olam Hesed Yibaneh
Our sages said that, Megilat Ruth, contains no new laws or matters pertaining to
our halacha. This megilah was solely written to teach the reward for those who perform
acts of loving-kindness. Ruth exemplified this attribute of "hesed," by giving up
her family and nation in order to selflessly help her mother-in-law Naomi. She did
so without any thought of her own needs, in a true act of hesed, despite being from
the nation of Moab (a hated and abhorred nation by the Jewish people of the time).
Ruth accepted the Torah without expecting anything in return. Similarly, on Shabouot,
our forefathers accepted the Torah on Har Sinai without seeking anything in return,
as they proclaimed during that eternal historical moment, "NaAseh VeNishma."
As a reward for Ruth's selfless conversion and acceptance of the Torah, she merited
being the grandmother of the greatest king of Israel, King David, from whom will
descend Mashiah Sidqenu.
3. Ru"th Misvot
When the Torah was given on Har Sinai, Bnei Yisrael were essentially given 606 Misvot.
They already knew the 7 misvot of Bnei Noah. Taf Resh Vav in gematria is 606 and
also spells the name "Ruth" in Hebrew. Therefore, because of this remez (hint) connecting
Ruth to the giving of the Torah, we read this megilah on Shabouot.