Happy New Year, Father Clavius!

According This Day in History, New Year's Day was celebrated on January 1 for the first time in 45 B.C.

However, the January 1 celebration fell out of practice during the Middle Ages because Julian calendar (named after Julius Caesar) followed the lunar cycle and was as much as 10 days off.

In the 1570s, Pope Gregory XIII commissioned a Jesuit astronomer, Christopher Clavius, to come up with a new calendar to better align the calendar with the solar year. This modification to the Julian calendar became known as the Gregorian calendar.

As a result, people once again began to celebrate January 1 in accordance with the more accurate Gregorian calendar.

In 1982, Father Cristopher Clavius, and the Gregorian calendar which he helped create, were commemorated on the Vatican souvenir sheet shown above.

To learn more, click here.