So, why now? Why aren't we celebrating Christmas in April, or July, or September?
The official explanation, of course, is that it's the day on which Jesus was born, but the evidence for that is worse than flimsy, consisting of two flatly contradictory stories in the New Testament. Now, I should make it clear that I'm largely neutral on how reliable the stories in the Gospels are, though I see the greater part, without commitment on the theological aspects, at least plausible as an outline of events and the closest we have to a near-contemporary account.
The nativity stories seem to me the least reliable part (they give contradictory accounts and don't convince me at all as stories) but again it's the closest there is to evidence. Nowhere does either account mention the time of year, but two details strongly suggest that it wasn't at the time we celebrate Christmas. For one thing, if the Romans were taking a census, it's unlikely that it would be done at the time of Saturnalia, their mid-December festival which seems to have strongly influenced Christmas.
The one dating hint in the tale is the detail that the shepherds were watching their flocks by night. There might be reasons for doing so at any time of year (persistent predators, for instance) but the only time when it would be normal would be during lambing. And that's in early spring.
Not that it really matters. It could be seen as an "official birthday", such as the Queen has (during the summer, so there's more chance of fine weather for the pageantry) and I suspect most Christians would say that what happened is much more important than when it happened.
But why midwinter? And why are so many other festivals held at the same time?
I suspect the reasons divide into symbolic and practical, though these might not be as separate as might be assumed. Much traditional religion was practical, based on the needs of the farming year, but that didn't make it any less important and significant.
And why should it? I remember seeing a piece on a TV programme years back (I've no memory of what the programme was) about a couple whose house in the country was on the north side of a mountain, meaning that they didn't see the sun at all from their home for several months. Each year, they held a little celebration on the first day in spring when the sun was visible.
Now, these were modern people. They had lighting and heating, sophisticated communications, and they had access to shops from which they could expect to get food at any time of year, but they instinctively sensed the significance of this moment. How much more important would it be to people who'd shivered in the cold and dark all winter, who knew they'd starve to death if the seasons somehow failed?
Festivals celebrating the sun's return, of course, belong to early spring, and our version of that is Easter (certainly at the right point of the year this time, since the events are specified as coinciding with Passover). But why would anyone choose to celebrate the darkest time of the whole year?
Part of the reason, as I said, is purely practical. In a subsistence farming society, early-to-mid winter is the time when all the harvests have been brought in and stored, the beer and cider have been made, the beasts that aren't going to be kept through the winter have been slaughtered and cured, probably salted. None of this would last very long, though, so it needed to be eaten and drunk. Add to this that it's a period when the farm only has to be kept ticking over, and the weather's bad — what better time to sit indoors around the fire, eating and drinking and making merry, singing songs and playing games, telling stories about the dangers safely outside in the darkness?
Then it gets harder. Work starts to build up again: in Britain, the first ploughing of the year is traditionally in January, and then, as we've seen, comes lambing. And the supplies dwindle. It might seem counterintuitive, but the hungry time is spring and early summer, when the food's gone and the harvests haven't started coming in yet. Midwinter is a time of plenty — provided, of course, that the harvest hasn't failed. In that case, all bets are off.
It probably wasn't lost, anyway, on even primitive peoples that the darkest time of the year is, paradoxically, the time when it all starts getting better. Not obviously at once, but this is where the sun begins to return, little by little, and many cultures have celebrated the birth of a little light in the midst of darkness, which would grow and eventually banish the dark. Symbolically, the perfect time to celebrate the birth of a saviour such as Christ or Mithras.
Christianity grew largely within the Roman Empire — either the western empire centred on Rome or the eastern empire centred on Constantinople — and it adopted many Roman customs and celebrations. The Roman midwinter festival of Saturnalia shared many characteristics with Christmas: feasting, gift-giving and the Misrule that was a common factor of Christmas till a few centuries ago.
Does it matter? I don't have any religious or emotional investment in this, one way or the other, but I think there are two ways of looking at it. On the one hand, the right symbolism is a powerful thing, and there seems no reason why it shouldn't be reused, especially when it coincides with the practicalities. On the other, if you have a strong investment in your version of the festival being the true one, encoded into the entire nature of the world — well, why wouldn't that truth and symbolism be felt incompletely by other cultures, other religions, even before the event? We're all a part of the same world.
Whatever festival you choose to celebrate at midwinter, may it be happy and filled with love. That, after all, is the important thing.