Why is the ninth month called September?
Old English geol, geola "Christmas Day, Christmastide," from Old Norse jol (plural), a heathen feast, later taken over by Christianity, of unknown origin.
The Old English (Anglian) cognate giuli was the Anglo-Saxons' name for a two-month midwinter season corresponding to Roman December and January, a time of important feasts but not itself a festival. After conversion to Christianity it narrowed to mean "the 12-day feast of the Nativity" (which began Dec. 25), but was replaced by Christmas by 11c., except in the northeast (areas of Danish settlement), where it remained the usual word.
Revived 19c. by writers to mean "the Christmas of 'Merrie England.' " First direct reference to the Yule log is 17c. Old Norse jol seems to have been borrowed in Old French as jolif, hence Modern French joli "pretty, nice," originally "festive" (see jolly).