In many synagogues the Book of Ruth is read on the second day of Shavuot. There are several reasons for this custom:

A) Shavuot is the birthday and yahrtzeit (day of passing) of King David, and the Book of Ruth records his ancestry. Ruth and her husband Boaz were King

B) The scenes of harvesting described in the book of Ruth are appropriate to the Festival of Harvest.

C) Ruth was a sincere convert who embraced Judaism with all her heart. On Shavuot all Jews were converts — having accepted the Torah and all of its precepts.