Its a tricky one. I was hoping Sunwell would be out around Christmas but it seems they aren't even thinking about it yet from the blue posts over the last week. I have to wonder whether TBC is stalling for the new expansion, or if the expansion is being stalled by the fact 2.4 still has to be tested, released and then live for at least 3 or 4 months before they can honestly release another expansion.
But if you look at the timescales involved it paints WotLK at a Q2-Q3 release date next year rather than the Q1 that would be abiding by their one a year policy (that didn't go so well last time either).
On topic: I believe we'll probably still clear BT each week and maybe Hyjal and do the odd attunement batch for players we've got to replace players leaving in the future (military service etc...).
Zul'Aman will certainly be popular during this period though, and a move of Kara to a 3 day reset would help to keep people entertained too. For guilds still learning content it could be tough though to keep up some serious progress nights during farm nights, my old guild had the same issue in the summer.
But I tend to think that this is less of an issue during the winter than it is the summer. Its cold outside and people want to stay in. Last years Winter lull was in my opinion almost solely to do with TBC and the fact that PvP epics were changed to welfare epics a few months before it. Kind of made serious progress in nax a bit pointless for anyone but the hardcore, and few guilds had 40+ who fit that description.
This time around we don't have WotLK looming so large on the horizon as we know we still have the buffer patch of 2.4 that you have to let people experience for enough time at 70 before wiping it out with a patch. Theres also the fact there won't be any "gear reset" on the scale of TBC. TBC's reset was in large fact due to the re-evaluation of the price of stamina on items so that TBC items had like double the stamina of non-TBC items. This, combined with two years of experience learning what each class/spec liked meant that new items were far better itemised than 60 blues (cloth items with no heal/dmg for example). Also this year everyone who has an hour a week can get access to T5-6 gear from the Arena so theres no need to bump up casuals with very good, very early gear to make sure that they can balance leveling content appropriately this time like there was in vanilla to TBC. This will hopefully mean that people will continue raiding as they don't feel the need to mass honour grind to catch the Nax farmers in gear for leveling in the expansion. Most of hte players who raid for gear have been rooted out now because Arena offers such an easier and less time consuming method of attaining similar levelled gear.
I wouldn't be so sure about the Christmas day raid, last year we managed a guild ZG run on christmas day due to the fact people wanted to escape nagging relatives or whatever

But in general yes declaring days you know will be a flop as off days is great because it doesn't look like a lot of people are just being lazy or slacking to those who do show up. We don't have the whole thanksgiving malarky over here so I can't say about that but I know you Americans are crazy for your holildays and do special things on each whereas Europeans, or certainly Brits, just tend to see them as a day off work except maybe Christmas/New years eve.
Pulling back from advanced raiding progress is a tricky and delicate issue. I don't know your progress level but regressing in content can be demoralising and depressing on those who are still raiding. Nobody wants to go from mid T6 back to gruul farming for a few weeks. I'd advise setting a level close to where you are at the moment and just step back one or two bosses to the ones you don't have much trouble with now.
Organise a couple of events like a ZA timed run. Two groups competing against each other for the quickest ZA clear. Balance teams (no officer/core group like our guild has

) and make sure no one has an advantage. The winning team gets free crimson spinels/bags of gold/guild ranks/
[Delicious Chocolate Cake] whatever.
Theres also the question of recruiting and how liberal or cut throat your guild is in this regards. If you're a tight knit small guild this probably won't be an option unless you're happy just removing the applicants once the lull ends. For a bigger and/or more casual guild mass recruiting to the tune of one or two extra per class would be an easy enough way to survive the lull and maybe even find yourself some new core raiders. Unfortunately for those past the Kael barrier this isn't really an option due to the attunement. Then again, if you're having a lull you may find yourself back in SSC/TK.
Theres also the option of running server competitions or selling raid spots to fill up the raid. Attunement runs to SSC/TK can be very popular if you're capable of it, just make sure the buyers aren't idiots or they could cause a night of misery for you and the other buyers on Kael/Vashj. These can fill in 2-3 spots of your raid if you're short, not ideal but arguably better than not raiding.
Also alt runs aint a bad idea if you can get the man power. Two problems arise there though, one is that very few seem to roll tanking alts, probably as its the only role with no potential outside instance groups. This means your tanks must still go on their mains and you risk tank burn out which is gonna hurt your progression more than not running the alt run. The second is that if your guild is like any other guild i've been in, the level of alts will differ widlly. You'll get some players who've got their mage in full spellstrike/spellfire/S2 gladiator weapon etc... gear whilst some will be wearing quest greens to 70. This may be okay for Gruul but the harder bosses in SSC won't cut it as they'll get gibbed by stuff non-stop.
A very risky stratergy is declaring a no raid period where members have no attendance commitments and can chill for the holidays for 2 or 3 weeks. This is so risky if you aren't your server's top guild though because they'll be going through a lull and likely won't think twice about stealing one or two of your most hardcore players who really want to raid during the holiday period.