twelfth (adj., n.)
"next in order after the eleventh; an ordinal numeral; being one of twelve equal parts into which a whole is regarded as divided;" late 14c., with -th (1), altering Middle English twelfte, from Old English twelfta, from twelf (see twelve). The earlier form is cognate with Old Norse tolfti, Danish tolvte, Old Frisian twelefta, Dutch twaalfde, Old High German zwelifto, German zwölfte .
As a noun meaning "a twelfth part," from 1550s. Twelfth Night is Old English twelftan niht "Twelfth Night," the eve of Epiphany, which comes twelve days after Christmas, formerly an occasion of social rites and a time of merrymaking.