gibsongirl, I cannot speak for what will be open on Christmas or New Years Eve but I will add my vote on restaurants already mentioned.
I think the best meal I've had in almost 10 years of visiting AC would be Rojo Lounge. SFJeff's menu/talent is outstanding (he was showcased with Andrew Zimmern on Bizarre foods). My second favorite meal came from Wild Mango. Next came Hidden Treasures (although the blood I lost from mozzie's may have made me a bit giddy and light headed).
Other restaurants that were a cut above for me were Capricorn (back in 2001) and El Divino. I have no doubt that where every you choose to spend those evenings will be spectacular. After all, you will be in my favorite place - Ambergris Caye. Enjoy!