Why is the ninth month called September?
Old English Wodnesdæg "Woden's day," a Germanic loan-translation of Latin dies Mercurii "day of Mercury" (cf. Old Norse Oðinsdagr, Swedish Onsdag, Old Frisian Wonsdei, Middle Dutch Wudensdach). For Woden, see Odin.
Contracted pronunciation is recorded from 15c. The Odin-based name is missing in German (mittwoch, from Old High German mittwocha, literally "mid-week"), probably by influence of Gothic, which seems to have adopted a pure ecclesiastical (i.e. non-astrological) week from Greek missionaries. The Gothic model also seems to be the source of Polish środa, Russian sreda "Wednesday," literally "middle."