In these situations is appropriate, I dare say good, to feel sorrow and grief in the midst of joyful celebrations. A person should not feel like they have to put on a cheerful face for everyone around them, hiding their pain in the hopes that it will just go away. Rather, taking the time to confront the loss--that abyss that stands in the place of such an important person--is necessary to keeping one's sanity.
It is also appropriate, and again good, to experience joy in the shadow of such grief. It is fine to forget and immerse oneself in the present. When those times come, and the feeling of guilt at having "forgotten" one's loved one begins to grow, it is best to note that those who have moved on to the next life are not making any claims on us. If anything, they want their loved ones to be comforted by the memories of the times they had together.
In times like this, we need to reach out to one another and relish the time that we have with those around us. This is the time to recognize that there are people missing in our lives, to acknowledge them, to celebrate the time we had with them, and to lean on those who remember them with us. It will not, and should not, take away the pain, but it will help keep them as a part of our holidays.