The world at that time used the Julian calendar. But its calculation of time was slightly off when it was adopted, which added up to ten days by 1582. The spring solstice was falling, by the calendar, ten days too early. In that year Pope Gregory chopped ten days from the calendar and instituted other changes (such as leap years not falling on certain century years) to bring the calendar in line with the seasons.
The Catholic countries of Europe adopted the system immediately, but the Protestants wanted nothing to do with any Pope's proclamation--until the 1700s, when things were now 11 days off and posing problems. In 1752 in England and in the American colonies, September 2 became September 14. Many people rioted, demanding their 11 days back. (Think of it: rents and other monthly bills would now be due 11 days sooner.)
More about this calendar business. (And a scientific explanation.)
This produced an interesting effect in England re Christmas, related to the story of the Glastonbury Thorn:
"The Glastonbury thorn legend ties in Christ's death as well as the celebration of his birth. The legend goes that soon after the death of Christ, Joseph of Arimathea came to Britain to spread the message of Christianity. When he traveled there from the Holy Land he brought with him his staff. Being tired from his journey, he lay down to rest. In doing so, he pushed his staff into the ground beside him. When he awoke, he found that the staff had taken root and begun to grow and blossom. It is said he left it there and it has flowered every Christmas and every spring . It is also said that a puritan trying to cut down the tree was blinded by a spllinter of the wood before he could do so. The original thorn did eventually die but not before many cuttings had been taken. It is one of these very cuttings which is in the grounds of Glastonbury Abbey today."Note the reference to "flowered every Christmas." With Pope Gregory's calendar reform, the thorn that bloomed on Christmas "old style" now bloomed on January 5, so many people refused to believe in this "new" Christmas and continued to celebrate the holiday in January.
Here's more about the story.
And a link to Glastonbury Abbey itself.
Some Eastern Orthodox churches did not accept the calendar reform; therefore in those churches today is Christmas Day, with Epiphany falling on January 15.
Epiphany and "Old Christmas" have nothing to do with each other.
Here are some other Old Christmas links, several from the Appalachians, where people celebrated Old Christmas for years after the calendar change.
A British site, Christmas-Time's entry.
Telleco Hills "Old Christmas Eve".
Chuck Larkin's Old Christmas page, with links to other Appalachian Christmas tales.
"Old Buck," an Outer Banks Old Christmas tradition, with a photo of the celebration from the 1940s.
And here are some Christmas sites I hadn't seen before:
An antique Christmas ornament collector's sitecheck out the Gallery link for photos of beautiful antique ornaments.
Fashion-Era's Christmas Pagesthis is a website of fashions; however, the Christmas pages also contain text, stories, and recipes associated with Christmas.
Also check out the entire Christmas-Time site.