Year 420 (CDXX) was a leap year starting on Thursday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Theodosius and Constantius (or, less frequently, year 1173 Ab urbe condita). The denomination 420 for this year has been used since the early medieval period, 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.