That the Catholics really have no idea when Jesus died. Same as how no one really knows what day he was born either. The Romans probably just picked December 25th because it was close to winter solstice, which was a holiday a lot of the pagans already celebrated. If it was even the Romans who decided what day Christmas was going to be. I'm not really sure about that.
It was the Roman emperor Constantine I.
Flavius Valerius Aurelius Constantinus, better known as Constantine I or Constantine the Great, was born around the year A.D. 275 - almost three centuries after Jesus. He worshipped the pantheon of Roman gods as all his predecessors had done and especially the Roman sun god, Sol. However, as he was traveling to battle his most powerful rival in Italy, Maxentius, at the Tiber River in A.D. 312, he had a vision. In that vision, he reported seeing the cross of Christ superimposed on the sun with the words, in hoc signo vinces - "in this sign you shall conquer." After winning the battle, he became a strong champion of Christianity. The very next year, he met with Emperor Licinius, the ruler of the eastern provinces, to sign the Edict of Milan giving equal rights to all religious groups within the Roman Empire. He returned property seized from Christians, built a great number of churches, donated land, and convened the first Council of Nicaea in A.D. 325 to deal with false teaching within the church. Although he didn't completely leave his pagan roots and wasn't baptized until A.D. 337 on this deathbed, he did much to further the growth of the church.
Either as a means to unify his empire, or to make converting to Christianity easier, Constantine sought to blend Christian and pagan traditions. At that time, two prominent pagan winter festivals were celebrated. The first, starting on December 17 and lasting seven days, honored Saturn, the Roman god of agriculture. The second, starting on December 25 and lasting through January 1, commemorated the birth of Mithras, the Persian god of light. Constantine merged many of the traditions from these festivals with the Nativity story in the Bible and Christmas was born. From its beginning, Christmas was a holiday (or holy day), gifts were exchanged, families and friends gathered to feast, and a birth was celebrated; just like in the Roman and Persian festivities.
The first mention of December 25 as the date of Jesus' birth is found in an early Roman calendar from A.D. 336.