Why is the ninth month called September?
late 14c., from Late Latin diurnalis "daily," from Latin dies "day" + -urnus, an adjectival suffix denoting time (cf. hibernus "wintery"). Dies "day" is from PIE root *dyeu- "to shine" (cf. Sanskrit diva "by day," Welsh diw, Breton deiz "day;" Armenian tiw; Lithuanian diena; Old Church Slavonic dini, Polish dzień, Russian den), literally "to shine" (cf. Greek delos "clear;" Latin deus, Sanskrit deva "god," literally "shining one;" Avestan dava- "spirit, demon;" Lithuanian devas, Old Norse tivar "gods;" Old English Tig, genitive Tiwes, see Tuesday).
diurnal di·ur·nal (dī-ûr'nəl)
Having a 24-hour period or cycle; daily.
Occurring or active during the daytime rather than at night.