quinzain

from The Century Dictionary.

• noun In chronology, the fourteenth day after a feast-day, or the fifteenth if the day of the feast is included.
• noun A stanza consisting of fifteen lines.
• noun A period of fifteen days (in modern calculation fourteen, leaving out the day of the feast) after a church festival, on the principle of the octave of a feast: as, the quinzain of Easter.
• noun Some event, social or other, which occurs every two weeks.