The monk who, in the sixth century, set the birth of Christ to the year 753, year of the foundation of Rom, made a miscalculation. However nobody proposes to correct that historical mistake nowadays.
One of the many needs of man is to list, order and date past significant happenings and events. But such a task requires a benchmark, or starting point to begin the counting.
For Christians, and therefore the entire Western World, this point is the date when Christ was born. That moment, of course, could not be established in the early days of Christianity, when it was barely known or spread. Actually it took longer to set that date. By the early sixth century, Christianity had spread throughout the whole former Roman Empire. At that time the political structures of the Empire had disappeared and the reference to the founding of Rome had less meaning than before.
Dionysius Exiguus (“exiguus” means small), a monk and astronomer born in Scythia, a region in southwest of present-day Russia, endeavoured thought to enhance and successfully did so the figure of Jesus by attempting to calculate the year of Christ’s birth and later setting that date as a reference. For Dionysius, this reference date became the “year zero” or the starting point of history. As such, the date had the effect of setting the beginning of the "Christian era". Ultimately, Dionysius concluded that Jesus was born the year 753 after the founding of Rome. In sum, there is the success of the Christian calendar, which is now almost universally accepted.
But now there is little doubt that Dionysius Exiguus is primarily responsible for erred in calculating Christ’s birth by approximately four years. At present, this mistake has still yet to be corrected. In support of this conclusion using other dates we also know that Jesus was born during the reign of Herod. However Herod died in the year 750 of Rome, therefore Jesus must have been born before such date. The resulting error miscalculates Christ’s birth day by approximately four years, an error that had yet to be changed to reflect its inaccuracy.
Currently, few people are interested in correcting the mistake, partly due to disadvantages and problems caused by the passage of time. For instance, can anyone imagine the billions of documents that would needed to be updated?
Yet, celebrating Christ’s birthday on December 25th is another mistake. At first it was held on January 6th and it actually continues to be commemorated on that date in the Eastern Church. What happened was that, as in many other occasions, a prior celebration was Christianized: "the day of the unconquered sun" which was celebrated on December 25 the winter solstice, when the sun, as newborn, begins to lengthen the day and decrease the night.
Such a date is noteworthy each year because the winter solstice is the precise date when the newborn sun arises and days begin to lengthen as a result of increasing sunlight.