Thirtnight 

A period of exactly thirteen days.

A thirtnight had passed after the great storm.

Thirtnight by MUSELUS89 Swagga November 5, 2022

thursnight

Thursnight - on a Thursday night, when you feel the urge to do something, but nothing as important as a weekend night.

Guy 1: "Man, it's only Thursday but I feel like doing something"
Guy 2: "Let's pick up some beers and play Rock Band and make it a thursnight"

thursnight by Trickyicky August 26, 2009

tortnight 

period of 21 days; period of 3 weeks. (1 tortnight = 21 days = 3 weeks)

The play will start in 1 tortnight.

tortnight by Chafe Ball June 16, 2018