I've previously discussed how long ago the universe began. Now, if the universe were purely physical, then this would be an one-dimensional question with a simple answer. But because of the depth and purpose expressed in the universe, multiple answers are possible, some more meaningful than others.
To see what I mean, let's look at a movie for an example. I take it that if you're here on my blog, you've seen Disney's "Frozen"? If not, go watch it - it's a good movie. Now, to the question: when did "Frozen" begin? One might answer that it starts when Elsa and Anna were kids, with the "Frozen Heart" song being sung by ice harvesters in the distance. Or, one might note that the oldest event referenced in the movie is the previous time someone was struck with ice powers, mentioned in the troll book. Or you can say that the movie itself was released in 2013. Or you can cite that development first began way back in 1937, when Walt Disney himself wanted to make a movie based on Hans Christian Anderson's "The Snow Queen". Or you may choose the publication date of "The Snow Queen" itself, back in 1844.
But, if I had to choose a moment in time for the "beginning" of Frozen, I would perhaps choose when "Let It Go" was composed. While this song was originally written for a villainous Elsa, it expressed such a relatable and sympathetic side to her that they had to re-think her whole character, giving us our Elsa as we know her. So it came to be that while "Let It Go" expresses Elsa's possibility for damnation within the story, it also turned out for her salvation in the story development.
This made Elsa the most memorable and human character in the movie, whose struggles and redemption infuses the whole film with that human element that makes "Frozen" what it is. Elsa, and therefore "Frozen" as a whole, cannot exist as they are without "Let It Go". That is why I say that if I had to choose a time when "Frozen" began, it began with the composition of that song.
Here's another example, this time from J.R.R. Tolkien's literary works: "The Silmarillion" tells the back-story of "The Lord of the Rings", starting with the creation story. In the beginning, Eru Ilúvatar - the One Father of All - creates the world with the great music of the Ainur. And yet, for a while, the music only remains music, or a vision, or a plan; it is not yet real, although it encodes all the events in all time for that universe. So Eru kindles the music with his Imperishable Flame, his Secret Fire, the one that is with Eru alone. It burns at the heart of the world, and that, finally, creates the World that Is. That is when that universe began.
What about our world? When did the universe begin? You can say that it started with the Big Bang, when the universe physically began about 13.8 billion years ago. You can say that it started when God breathed life into Adam, who's often thought to have lived about 6000 years ago. These are not wrong answers. But I do think that there is a more meaningful answer still.
What is the purpose of the world? The world exists by, for, and through Jesus Christ. It was created to allow Christ to be incarnated into it, so that he can carry out his eternal plan of salvation through it. Jesus truly is the Creator and the Firstborn over all creation; for without him, nothing would exist, and there is nothing that exists ontologically before him.
So that is the significance of Christmas day: it is when God himself sent his Son into the universe to make it real. All other times, all other events, and all other things - both forwards and backwards in history - orient themselves with respect to the Son of God and are made real by their connection with him. And he - the Creator, Sustainer, and the Firstborn of all creation - came in contact with us upon his incarnation, being born to Mary and Joseph in a manger.
Merry Christmas to you all - for on that day the universe was (ontologically, not temporally) created.
You may next want to read:
For Christmas: the Incarnation
The biblical timeline of the universe
Another post, from the table of contents