My family celebrates Chalica,which is seven days long and begins on the first Monday in December. Each day represents a different Unitarian Universalist Principle; a chalice is lit each day and actions/gifts/volunteering may be given and received. We also celebrate Yule, which is a celebration of the winter solstice and Christmas. December is a really busy month for us! Why do we celebrate these holidays? Chalica, because we belong to the local UU church and I believe the seven principles are important to follow, Yule, because as a pagan I follow the sabbats and to me Yule brings the rebirth of the SUN, Christmas is celebrated here because it is a fun way to share fun and festivities with our non pagan family.
There are 2 other Holidays celebrated this month, Hanukkah which is a Jewish celebration and Kwanzaa which is a week long celebration held in the US honoring universal African heritage and culture. So, I as a Pagan can greet everyone with Merry Yule and someone else can call out Merry Christmas and we are both assuming that the people we are greeting hold the same beliefs as we do when in fact they may believe in one of the other Holidays or in none at all. I think that Happy Holidays would be the most respectful greeting unless you know for sure otherwise.
Happy Holidays to all and Blessed Be!