This would really depend on how old the piano is, and when it was last tuned. If it has been tuned in the last few months, then it should be more or less on concert pitch, and shouldn't present any problems with tuning. We would recommend in such a case that the piano is left to settle in its' new environment for around a week or two, and then it will be ready for a fine tune. There shouldn't be a problem with the piano holding its' tuning for several weeks, or until after Christmas, providing it was close to pitch when it was moved.

The problems arise with a piano which has been neglected, and which has missed several tunings over the last couple or more years. If a piano is not tuned for a while, it will normally start to go flat of concert pitch. Not only will this make it impossible to play the instrument with another musical instrument which cannot be tuned to the piano (i.e. a brass or woodwind instrument), but the piano will start to lose its 'brightness' in sound, and could sound quite dull in tone. The difficulties in the tuning of such an instrument come with raising the pitch to where it should be. If it has gone too far flat, then as the strings are pulled up to pitch, the whole piano will start to pull itself flat again, making it impossible to put the piano in tune with just one tuning.

In this case, we would recommend that the piano is brought up to pitch with a pitch raise and a rough tune, and then left for at least a week to settle into its' correct pitch. After this settling period, the piano will be able to take and hold a fine tune, which should stay pretty much in tune for several months, providing that the instrument is in good shape.

Therefore, with regards to having a piano tuned in time for Christmas, a good two or three weeks before should see that you have a well-tuned piano for over the Christmas period, in most cases. But if you're not sure whether or not it is on pitch, then you really shouldn't leave it until the last minute, or you could find yourself with an out-of-tune piano for those Christmas parties, which will make those carols sound not quite as lovely as they should!
A Piano will usually stay in tune if it is gently moved from one side of a room to another, but after a move such as in a van, the bumps and jolts will inevitably knock it out of tune to some degree. 

However, the biggest concern with moving any piano is the climate change from the room in which it has been situated, to the new room in which it will be going. Even if it is in the same house! But the further a piano travels, the more chance of a change in climate, which the piano will feel, and respond to. The whole soundboard will change shape slightly, therefore changing the tension on the strings. This will result in a 'settling-in' period, of a week or two, or maybe a few months, where the stability of the tuning may well suffer.

If we were to take, for instance, a piano which has just been tuned, from a Centrally-heated (dry heat) home, to a cool but humid home, it would probably sound fine immediately after the move. But within a few days, after adjusting to the higher humidity, and temperature variations,  it will start to sound quite out of tune.

If the piano hasn't got far to go though, and if it is treated gently, as long as the climate from one place to another is pretty similar, the tuning should not really suffer too much. All pianos need to be tuned regularly to keep an even and correct tension throughout the instrument, but they are quite hardy things, and moving should not bother a piano, if done correctly.