You shall count for yourselves... from the day on which you bring the Omer offering, seven complete weeks they shall be... (Leviticus 23:15)
The word sefirah, "counting", also means "luminance". On each of the 49 days of the Sefirat HaOmer ("Counting of the Omer"), we refine, rarify, and illuminate another of the forty-nine traits of our souls.