Excerpts from Books and Wikipedia
Julius Caesar reformed the calendar based on the advice of the astronomer Sosigenes. The Julian calendar has a solar year of 365 and 1/4 days with an extra day every fourth year. The Julian calendar was replaced by the Gregorian calendar by Pope Gregory XIII in 1582. The average year is 365.2425 days in the Gregorian calendar versus 365.25 days in the Julian calendar. [Furtado: 1001 Days]