"Twelfth" refers to the number 12 in the sequence of counting. It is the ordinal form of the cardinal number 12, and it comes after the eleventh and before the thirteenth. Full definition
Indeed, the room in which I spoke (Parliament Chamber) hosted the very first performance of Twelfth Night in 1602. (slaw.ca)
Yet less than half of Asian American students were reading (English) at a college - ready level in twelfth grade. (educationnext.org)
Naucheta, Turkistan, Genda, Tangut and others, whence it will be manifested that there were a vast multitude of Christians in the eleventh and twelfth centuries in those countries, which are now devoted to Mohammadanism or the worship of imaginary gods. (religion-online.org)