With Christmas (and I can't believe I'm typing that word this early in the year!!!) you basically have 3 options.
1. If Christmas will only last one day - Relax, enjoy, accept the gain and know that it will soon come off in the New Year. That this New Year's 'I'll lose weight after Christmas' won't be just an empty resolution but an achievable goal with long term results.
However, if Christmas leads to boxing day, leads to New Years Eve, leads to New Years Day.... and you don't want too much of a mountain to climb in the New Year there is always option 2...
2. Plan, plan, plan. If you do a search of this site, last year someone named Phil (I think!!) ran a competition to plan a SW menu for Christmas day. It could be for any plan, had to include breakfast, lunch, dinner, desserts, snacks, drinks and only 15 syns. Even if you don't use all the ideas on Christmas day and you allow yourself some little indulgences, then at least you can help limit any gain. You might come up with even better menu ideas, and can use them over the Christmas week.
Or you can go for option 3.
3. Accept that Christmas day/week usually comes with a gain. If you think that you can still lose over the festive period whilst still enjoying yourself, go for it, it can be done. If you want to allow yourself some wriggle room over the week but you think that you would be disappointed to see a gain, this is what I did.....
Towards the end of November, pick a weeks weight that you are happy with and place your line in the sand. Any loss between that week and the start of the Christmas celebrations goes into a 'weight bank'. By the time you start celebrating, any loss in the 'weight bank' is yours to gain. The line in the sand? Helped me with the panic of 'If I have a gain, it'll just be the start of a slippery slope back to my starting weight'. Even if I was to put on over Christmas, I would never be that weight again.
I can see that you have only just started and are in the same mind-set that I was last year (I started on 2nd September 2009). The 'Christmas is coming!!! How will I cope??? It's all going to go horribly wrong and all of my good work will be for nothing!!! Why didn't I wait till after Christmas and just join in the New Year??!! Blues Panic'. Don't worry, most of us have had them at some point!!!
There will always be a reason why we should wait, Christmas, New Year, birthdays, anniversaries, parties etc, but just as many reasons to start Christmas, New Year, birthdays, anniversaries, parties etc we all want to look good, feel good and not feel that our weight is holding us back. Everyone on this site has made the greatest first step, now we just have to realise that this is our way of life, not a diet.
We will be like everyone else, enjoy life, eat a little too much occasionally, and then lose the little gains. I can't think of a single person whose weight stays perfectly level (and if there is, please don't post, I'm trying to make a point!!).
HTH