Why is George Washington's birth date, for example, shown sometimes as February 1731/32? He could have been born in only one year!
During the rule of Julius Caesar, the Romans adopted the calendar with leap years. This became known as the Julian Calendar after Julius Caesar. By the year 1582, the calendar was over ten days off. Pope Gregory II changed the calendar slightly to account for this difference by skipping the leap day in three of four century years. He did this because relgious holidays did not correspond to the season as they originally had been intended to.
Until this time, the Roman practice of beginning the year on March 15 (the Ides of March) was standard. Gregory adopted January 1 as the beginning of the New Year to correspond to the Christmas season and the birth of Jesus.
Not all countries adopted the Gregorian Calendar right away. England and its colonies did not do this until 1752. Until then, January, February, and the first two weeks of March still belonged to the old year. So when George Washington was born, the records were written that he was born February 11, 1731, because England and its colonies were still using the Julian Calendar. When the English adopted the Gregorian Calendar in 1752, that meant Washington's birthday under the new system was February 22, 1732. They added 11 days to make up for the time lost over the years, and they recalculated the year's first day to January 1.
Dates of years between 1582 and the time when the country adopted the new calendar are often written with a virgule to show the old and new notation.
Another historical note: Russia did not adopt the Gregorian Calendar until 1918. That is why the "Red October" Revolution of 1917 began November 7. That was October 24 according to the Julian Calendar.