Biannual maintenance duty was a period of repair mandated for Federation starships every six months under Starfleet regulations in the 2270s.
Considered the bane of all Starfleet personnel, it was performed while in drydock. It was thought to be a very tedious and thorough process.
History[]
In February 2274, the USS Enterprise put in at Hus-24 drydock for biannual maintenance duty. James T. Kirk chose the facility because Starfleet wanted the ship in that sector, but also wanted to give the crew shore leave on Hus-24. Kirk allowed Husians to perform repairs outside of the computer core and other sensitive areas of the ship. (TOS comic: "Husian Gambit")