Why is the ninth month called September?
Old English niht (West Saxon neaht, Anglian næht, neht) "night, darkness;" the vowel indicating that the modern word derives from oblique cases (genitive nihte, dative niht), from Proto-Germanic *nakht- (cf. Old Saxon and Old High German naht, Old Frisian and Dutch nacht, German Nacht, Old Norse natt, Gothic nahts).
The Germanic words are from PIE *nekwt- "night" (cf. Greek nuks "a night," Latin nox, Old Irish nochd, Sanskrit naktam "at night," Lithuanian naktis "night," Old Church Slavonic nosti, Russian noch', Welsh henoid "tonight"), according to Watkins, probably from a verbal root *neg- "to be dark, be night." For spelling with -gh- see fight.
The fact that the Aryans have a common name for night, but not for day (q.v.), is due to the fact that they reckoned by nights. [Weekley]Cf. German Weihnachten "Christmas." In early times, the day was held to begin at sunset, so Old English monanniht "Monday night" was the night before Monday, or what we would call Sunday night.