January 1

153 BC - Roman consuls begin their year in office
45 BC - The Julian calendar takes effect for the first time.
1 - Origin of Christian Era
990 - Russia adopts Julian calendar
1583 - 1st day of the Gregorian calendar in Holland & Flanders
1362 - Grand Duchy of Lithuania
1522 Republic of Venice 
1544 Holy Roman Empire (Germany)
1556 Spain, Portugal
1559 Prussia, Sweden
1564 France
1576 Southern Netherlands
1579 Lorraine
1583 United Provinces of the Netherlands (northern)
1600 Scotland
1700 Russia
1721 Tuscany
1752 Great Britain (excluding Scotland) and its colonies