We take calendars for granted. These days, your cell phone will tell you what day it is. And there's the calendar hanging on the wall, the announcement on the radio, the newspaper, etc.
But figuring out what day it was, or even what year it was, was not self-evident in the Middle Ages. They were working with the Julian Calendar, one of the few things Julius Caesar was responsible for in the brief interlude between becoming emperor and being assassinated on the Ides of March, the assassins' unrealized intent being to restore the Roman Republic.
The Julian Calendar recognized, as previous calendars had not, that the year is not exactly 365 days long, but rather 365 1/4. This is why we have "leap year" every four years, with an extra day. Before the institution of leap year, the calendar would creep, getting 1 more day out of synch with the seasons every four years. After a while, one starts to notice. Ancient Egypt, that had a 365-day calendar, had had a long enough civilization for festivals to work their way all the way around the year twice.
The Julian Calendar was thus a great improvement. But a year is still not exactly 365 1/4 days long. To make it work just right, you have to skip leap year every 100 years, but have leap year every 1000 years (we had leap year in the year 2000). So, although from the first century on they no longer had the problem of getting further out of synch with the seasons by one day every four years, they were still gradually getting out of synch by one day every century.
Thus, by the late Middle Ages, they were about two weeks off, with Christmas coming not within a few days of the winter solstice (shortest day of the year) but rather in what we think of as January.
Christmas carols from the Middle Ages and Renaissance have lines like, "In the deep midwinter, frosty winds made moan, earth as hard as iron, water like a stone." It's still bitterly cold in January in the northern hemisphere (at least in those parts that get cold), but we now assume Christmas is earlier, at the beginning of winter.
The modern calendar is called the Gregorian Calendar, pronounced by Pope Gregory XIII, who was concerned at Easter's drift away from the equinox. Protestant countries, including the American colonies, initially refused to recognize it, assuming it was some sort of papal plot. When the American calendar was finally changed, in the late eighteenth century, a number of people were distraught over their "missing" weeks.
Click here for more on telling time in the Middle Ages.