How does the catholic community know that Jesus was born on the 25th of December and not on some other day?

First, we will have to keep in mind that celebrating Christmas on December 25 by the Catholic Church is a matter of custom rather than doctrine. Therefore it should be noted that though the Church chooses this date, she is not declaring this should be held as an obligatory article of faith and that this is the literal date of Christ’s birth. Several dates were observed as the day Christ was born by the early church. Some of them being May 20, March 28, April 19, January 6 and December 25. 

The reason behind the selection of December 25 lies with its ancient origin, widespread acceptance and the evidence we see in the Scripture. St. Luke tells us that Zechariah, the father of John the Baptist, was of the priestly order of Abijah (Luke 1:5) and by analysis of the historical data available to us we can say with reasonable certainty that he was on duty somewhere in late September. The Scripture also tells us that John the Baptist was conceived immediately after Zechariah returned from the temple (Luke 1:24), putting the conception towards the end of September. If John was conceived in late September then we can also say that he was born nine months later towards the end of June. 

Why is this important? It is important because from the Gospel of Luke we also know that the Annunciation took place six months after John was conceived (Luke 1:26) and because of this John was six months older to Jesus Christ. So what happens when we add six months? We reach late December, the period when Jesus was born. 

We do have writings from early church fathers such as St. Theophilus (Bishop of Caesarea, Died 181 AD) and St. Hippolytus of Rome (Died 235 AD) who mention that Jesus was born on December 25th.

“We ought to celebrate the birthday of our Lord on what day soever the 25th of December shall happen.”- St. Theophilus 

“The first advent of our Lord in the flesh occurred when he was born in Bethlehem on December 25.” – St. Hippolytus of Rome in Commentary on Dan­iel (204 AD).

It should also be noted that the Annunciation was celebrated in the Church on March 25th since ancient times, placing the birth of Jesus nine months later on December 25th.

Church history, sacred tradition, and biblical scholarship all points towards December 25th as being the most probable day Jesus was born.

As we are discussing the validity of December 25th being the day Jesus was born, let’s also look into three common objections that are found making their rounds among both Christians and non-Christians. The first two put forth the idea that the Catholic Church baptised an already existing pagan festival (Saturnalia) and a pagan holiday (Natalis Solis Invicti – the birthday of the unconquered sun) in order to gain the upper hand. The third one appeals to the notion that it would be too cold during winter time to travel or to have shepherds tending to sheep outside in nearby fields. 

What we need to understand regarding these are as follows:

1. Saturnalia was celebrated between December 17th and December 23rd and it can be clearly seen that the dates do not match up. If the intention of the Catholic Church was to replace the pagan festival then fixing December 25th as Christmas doesn’t make any sense.
2. Though Sol Invictus was instituted in AD 274 it was not celebrated until AD 354 and even then it was known only as Invictus with no mention of a birthday. It only became the birthday of the unconquered sun during the time of Emperor Julian the Apostate, who rejected Christianity in favour of Paganism. It is clear from this that Emperor Julian was actually trying to paganise a Christian holiday and not the other way around.
3. The birth of Jesus took place in Palestine where the temperature during the winter season is mild. Therefore the whole premise that it would be too cold to travel or tend the sheep is false.