Back in the Middle Ages, Europeans held the end of one day and the beginning of another to occur during sunset. Thus, they considered Christmas to begin at sunset on what we would call December 24th. They got into the habit of holding two Christmas celebrations, one which they called Christmas Evening before going to bed, and another called Christmas Morning after waking up. Both were considered to be part of Christmas, since both occured on what they reckoned to be December 25th.
Eventually, though, mechanical twelve-hour clocks came into fashion, and it became customary to set the twelve o'clock hour at high noon, with the twelve hours before noon known as ante meridian and the twelve hours after noon as post meridian. This had the effect of moving the beginning and end of the day from sunset to midnight, so that was when December 25 was held to begin. However, people still referred to the period after sunset on what was now December 24th as Christmas Evening, and over time the term "Christmas Eve" came to refer to the day before Christmas, rather than the first half of Christmas.
Here on the east coast of North America, the sun has set, so it's now Christmas Eve by both the medieval and modern reckonings. That being the case, we here at the Johnny Pez blog now feel justified in wishing our hypothetical readers a merry Christmas.