I would break it off now. Waiting until after the holidays is only postponing the inevitable and living a lie. It's not worth it. Although you may feel that you are doing someone a favor by "getting through the holidays" together, it will eventually come out, and this will only serve to make things worse for everyone involved.

I would likely end the relationship immediately and hope that I would not ruin the poor guy’s future holiday season by reminding him of the breakup. However, I would think that it would be wrong to pretend to enjoy the holidays together as a couple when one of us is no longer happy in the relationship and has the intention of leaving. I would not want to give him false hopes or the opportunity to perhaps buy an expensive gift for me for Christmas (such as jewelry, or God forgive, an engagement ring) that I could then not accept without letting him know why. Sure it would ruin the man’s holidays this time and he may take it hard and not forgive me for it, especially if he’s the type of person who gets into a festive spirit and seeks closeness to loved ones during the holidays, but it would simply not feel right to lead him on. Further, many couples oftentimes make plans with their families for Christmas and spend the holidays at either their parents, sisters, or brothers house or that of their partner. Doing so would only give him the false perception that we would actually have a future, since he may bond with my family and expect to be part of it someday.
Therefore, I think letting the guy off easy by telling him how hard it was for you to make this decision but letting him know that you are sure it’s for the best, may after all be the most reasonable approach to ensure that neither of you is entrapped in a relationship that is not going anywhere.

I am with the others, as cruel as it is to wreak the holiday spirit of others, it must be done as soon as possible. Think of it this way you will be happier sooner rather than later, and you are not stressing out about it any longer.