General fact: 273 is the zero of the Celsius temperature scale (to the nearest whole number) in Kelvin.
Mathematical fact: 273 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Trivia: 273 is the number of seconds in American composer John Cage's piano composition 4'33" of silence, in three movements.
Historical date: September 29th is the day in 2004 that the Burt Rutan Ansari X Prize entry SpaceShipOne performs a successful spaceflight, the first of two required to win the prize.