Thirty is:

• It is a primorial as well as the sum of the squares of the integers 1, 2, 3 and 4. It is the smallest Giuga number. 30 is the smallest sphenic number
• The number of days in the months April, June, September and November (and in unusual circumstances February - see February 30)
• The total number of major and minor keys in Western tonal music, including enharmonic equivalents
• In years of marriage, the pearl wedding anniversary
• The duration in years of the Thirty Years' War
• Used to indicate the end of a wire service story. (Possibly a corruption of German fertig - "finished, ready" - or in journalistic context "end of story"); in several Superman stories from various titles, failure by a Daily Planet employee to use this signature proved to be a plot point revealing a character's impersonation, mind control, etc.
• The code for international direct dial phone calls to Greece
• The house number of 30 St Mary Axe (The Gherkin)
• The designation of Interstate 30, a freeway that runs from Texas to Arkansas
• The designation of U.S. Route 30, a highway that runs from Oregon to New Jersey
• Various other routes have been numbered "30"; for example, New York State Route 30 which runs from the Pennsylvania border to the Canadian border
• The designation of E30, the European route from Cork to Samara
• The number of tracks on The Beatles' eponymous album, usually known as The White Album
• The number of cars in the Nintendo 64 game F-Zero X
• -30-, a motion picture about work in a Los Angeles newspaper, starring Jack Webb and William Conrad
• A stage in young adulthood
• Part of the name of:
  • Thirty odd foot of grunts, the band fronted by actor Russell Crowe
  • The movie title 13 Going on 30, starring Jennifer Garner
  • The title of the Food Network show 30 Minute Meals
• The uniform number of Maury Wills when he played for the Los Angeles Dodgers.
• Judas Iscariot betrayed Jesus for 30 pieces of silver. Matthew 26:15.