Year 42 (XLII) was a common year starting on Monday (link will display the full calendar) of the Julian calendar. At the time it was known as the Year of the Consulship of Caesar and Piso (or, less frequently, year 795 Ab urbe condita). The denomination 42 for this year has been used since the Early Middle Ages, when the Anno Domini calendar era became the prevalent method in Europe for naming years.

Following the break-up of Czechoslovakia in 1993, the successor states, the Czech Republic and Slovakia, continued to share the 42 country code, until 28 February 1997, with the Czech Republic adopting 420 and Slovakia adopting 421.