If you live in the Northern Hemisphere, today is the Winter Solstice, which marks the shortest day and longest night of the year.
When St. Augustine preached his "Sermon 192" in the 5th century, he clearly was not thinking of Christians living below the Earth's equator -- where today is the height of the Sun's power at midsummer:
"... (Jesus Christ) was born on the day which is the shortest in our earthly reckoning and from which subsequent days begin to increase in length. He, therefore, who bent low and lifted us up chose the shortest day, yet the one whence light begins to increase. By such a coming, though silent, He urged us, as with the sound of a mighty voice, to learn how to be rich in Him who became poor for us; to accept freedom in Him who took the form of a slave for us; to possess heaven in Him who sprang from earth for us."
There is still some debate among Biblical scholars and historians about the real date of Jesus' birth, which some say was in Spring (the month is not mentioned in the Bible). Long pored over by the early Christians, there is evidence that the selection of December 25 as the date of Jesus' birth had little to do with pagan solstice festivals celebrated at the same time, but it is certainly true that pagan Sun worship became syncretized into Christmas celebrations, especially as the Church won new converts.
But as Christmas was a festival created in the Northern Hemisphere (the earliest record we have of its celebration is in the early 4th century in Rome), it's easy to see why the universal archetype of the Sun at Winter Solstice would be a natural and beautiful choice for the birthday of Jesus.