The first suggests that Christmas is really a midwinter festival, very northern European in its focus - the scenario of Nordic folk eating and drinking themselves into a sort of haze to while away the dark nights. Prince Albert's enthusiasm for Christmas trees is but part of that. And the boom decade we have just experienced made the family consumer spree even more adventurous than it had been before.
This is the explanation of popular legend - the one which is easiest to project and handle. It is also the one which is liable to cause irritation to the hyper-devout, who are likely to respond with killjoy talk of the 'real meaning of Christmas' as if that were a commodity just as easily packaged.
The second explanation probes a little further into history, and is a sort of scholarly variation on the previous one. Christmas, on this view, owes its origin to the early centuries of Christianity, when it was necessary to combat pagan practices. In the western Mediterranean, the midwinter solstice was 25 December, whereas in the eastern Mediterranean it is taken to have been on 6 January. When Christians began to settle into a proper liturgical year in the fourth century, they decided to 'Christianise' these two feasts.
In the West, there was the added need to do battle with the pagan festival of the unconquered sun, which was established on 25 December by the Emperor Aurelian in 274. Worship of the sun was no Roman prerogative, for we know that 'the sun of righteousness' (Malachi 4:2) was a motif representing the One True God over and against the sun gods of the Ancient Near East. 'Hail the sun of righteousness' has passed into popular Christian usage through the hymn 'Hark the herald angels sing'; at our annual carol service, I wear a cope with the star of righteousness embroidered on the back.
There is clearly much to commend this view. It places Christianity on the defensive, sorting out religious practices and beliefs that are not up to the mark. It places the God of Abraham, Isaac and Jacob, and of Jesus Christ, within human history, asserting his claim to be worshipped by all nations, and, in so doing, making space within the already-existing religious milieu of historic communities. It also explains why, for example, in a later century, the Nativity of John the Baptist was kept on 24 June - Midsummer's Day.
But there is a third explanation which has not so far gained the attention it deserves, and this is all the more surprising in that it has been around, albeit in embryonic form, since 1889, when it was first put forward by the French scholar Louis Duchesne. It has in recent years been considerably refined by the American scholar Thomas Talley, whose magnum opus on the origins of the Christian year appeared in 1986. (It has since been translated into French: how many other Anglican scholars have had that honour?)
The Duchesne-Talley view has been dubbed the 'calculation-theory'. On this basis, the choice of 27 December (in the West) and 6 January (in the East) is to do with calculating nine months away from the supposed date of the death of Christ. Early Christian writers - like the rabbis of old - loved symbolic dating, and they took the view that Christ's conception would have to take place on the same date as his crucifixion. The Jewish date for this, the Passover, was 14 Nisan, which in the old Julian Calendar was 25 March, but, in the version in use in the East, was 6 April. Some of these early Christian writers sound as if they would have spurned the 'personal organiser', but they loved playing around with dates.
Both these last two theories, of course, neatly explain why Western Christianity places the spotlight on 25 December and Eastern Christianity does so on 6 January. But they provoke quite different reactions. The 'solstice-Christianised' view is old, easy to grasp, and appeals to a Christianity that grapples with what it has to face. The 'calculation-view' is new, complicated to grasp, and speaks of a Church that is rather more self-confident, and, moreover, one which makes a vital link between Easter and Christmas.
Scholars will no doubt wrestle with such problems as these for a good time yet. My money is on the 'calculation-theory'. It has a ring of truth about it, even though as a father of four I am always intrigued at the assumption that the first-born arrived bang on time - aren't the first often notoriously late? It also reflects a Christianity that can stand on its own feet, and speak its message on terrain in which it will always be a bit of a stranger.
Above all, I like that link between the birth and the death of Christ, something which artists and poets often find to their liking. That wholesome picture, too, can live alongside anything - and that includes the stark contrast of those who have to sleep rough in the shadow of expensively locked department stores in our cities today.