What is the origin of the Jewish holy day Shavout?

Leviticus 23:15 "'From the day after the Sabbath (which is the day of the offering of the first-fruits), the day you brought the sheaf of the wave offering, count off seven full weeks. 16 Count off fifty days up to the day after the seventh Sabbath, and then present an offering of new grain to YHWH. 17 From wherever you live, bring two loaves made of two-tenths of an ephah of the finest flour, baked with yeast, as a wave offering of firstfruits to YHWH. 18 Present with this bread seven male lambs, each a year old and without defect, one young bull and two rams. They will be a burnt offering to YHWH, together with their grain offerings and drink offerings-a food offering, an aroma pleasing to YHWH. 19 Then sacrifice one male goat for a sin offering[c] and two lambs, each a year old, for a fellowship offering. 20 The priest is to wave the two lambs before YHWH as a wave offering, together with the bread of the firstfruits. They are a sacred offering to YHWH for the priest. 21 On that same day you are to proclaim a sacred assembly and do no regular work. This is to be a lasting ordinance for the generations to come, wherever you live."