The custom of reading the Book of Ruth on Shavuot is an ancient one, known to have existed already in the Gaonic Era (6th-11th centuries). This custom derives from the harvest atmosphere, against the backdrop of the Book.
The Book was probably written with the intention of creating an atmosphere conducive for receiving foreigners by invoking the image of the gentile woman, Ruth the Moabitess (from Moab), who adopts the Torah of Israel (“Your people will be my people and your God my God”). Ruth is destined to be grandmother of Yishai, father of David, King of Israel, symbol of national independence and the messianic vision.