Everything will be open unless there is a big drought (unlikely) or massive warm/rain event (pretty unlikely as well this year).
Last 2 seasons have been tons of fresh snow around Christmas week.
No idea about the state of the park, definitely something up on both mountains, but maybe not highest level one yet.
Line-ups will be insane as everyone has stated. Some tips:
1) Ski on New Years day if you are there, fight through the hangover, way smaller lines.
2) Be in line for the gondola before it opens (8:30), if you show up at 9 you will be waiting much longer, if you are first up, you get a couple quick laps before the mid mountain lifts start to line-up.
3) Do Fresh Tracks breakfast on Whistler if there is any new snow, $17 for a nice breakfast buffet and 30min on the mid mountain lifts before they start uploading. You will have to line-up at 7am or earlier though since you can buy tickets for any day (they let the first 100 or whatever the cap is up).
4) Don't go out of bounds unless you know the mountains already, it is very easy to get lost and it gets dark/cold early at that time of year.
