In B.C. 45, Julius Caesar introduced the Julian calendar (which had no intercalary year) and, similar to the Gregorian calendar, January 1st became New Year's Day (the first day of the new year).
紀元前45年、ユリウス・カエサルがユリウス暦（閏年が無い）を導入、1Januaryがグレゴリオ暦同様、正月（新年初日）になった。