20

Year 20 (XX) was a leap 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 Marcus Valerius Messalla Barbatus and Cotta (or, less frequently, year 773 Ab urbe condita). The denomination 20 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.

https://en.wikipedia.org/wiki/20