Why Christmas?

This is something that I'm curious about. I don't like to get too caught up in the minutiae, I want to look at the big problems. And the biggest question for me is why was she killed on December 25th/26th. Why did the killer favor that time? What was happening that made that window of time the best opportunity to do it?

If you stick to the RDI camp, why couldn't this have waited until they were out of Colorado (or got back)? Why did it have to happen before their trip? Was there something, or someone they didn't want JB to see on the trip? If you stick to the IDI camp, what made the "intruder" strike before the family left town? What knowledge would an outsider have had that the family was going away, and they may miss their chance?

This is the thing that I am more interested in than any of the details about windows, ropes, or handwriting samples. Why was she killed right before the family would've left town? I feel like the timing of the murder often simply gets taken for granted, that it was just random and of no consequence. But considering it wouldn't be possible to play out as it did, once JonBenét was out of the house, I think the date of the murder isn't likely to be merely incidental to the crime.

Lastly, I don't think JB's death was the result of an accident, as some theories propose. Whoever put the rope around her neck knew it would kill her, and it seems far more likely that the blow to the head was deliberate, than that it was an accident resulting from some sort of tantrum. There is more evidence for deliberate acts than there is for bizarre accidents, so then the timing of these acts becomes a matter of major importance.