Why is the ninth month called September?
early 15c., "full of light," from Latin luminosus "shining, full of light," from lumen (genitive luminis) "light," related to lucere "to shine" (see light (n.)). Related: Luminously.
luminous lu·mi·nous (lōō'mə-nəs)adj. Emitting light, especially emitting self-generated light.