The year 46 BC had 445 days because Julius Caesar’s calendar reform had to repair years of accumulated drift. It became known as the annus confusionis, or “Year of Confusion,” and is generally described as the longest calendar year in recorded history.
The old Roman calendar was based on lunar months but was periodically adjusted with an intercalary month to keep it aligned with the seasons. Political and civil disorder had caused these corrections to be neglected. By Caesar’s time, the calendar was badly out of step with the solar year.
To realign the calendar before the new system began in 45 BC, Caesar inserted extra time into 46 BC. The year already contained the normal Roman calendar structure and an intercalary month; Caesar then added two extraordinary intercalary months between November and December. Together, these additions brought the total to 445 days.
The Julian calendar took effect in 45 BC and established a 365-day year with a leap day every fourth year. It remained the dominant calendar in much of Europe for more than 1,600 years, until the Gregorian reform began replacing it in 1582. The 445-day year was a one-time correction, not a normal feature of the Julian calendar.