How many days were in a common year in the Julian calendar?

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Kalendarium (“Calendar”) by Regiomontanus — calendar First complete printed title page for the *Kalendarium* (“Calendar”) by Regiomontanus, 1476.

In the Julian calendar, a common year consists of 365 days.1

This system, established by Julius Caesar and designed by the astronomer Sosigenes, accounts for the solar year by incorporating a leap year of 366 days every four years. In a common year, the month of February contains 28 days, whereas it contains 29 days during a leap year.21