All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is permissible for you to visit your mother at any given day, however, it is more appropriate for you to avoid visiting her on Christmas day, but if your visit coincides with Christmas, then you are not sinful if such visit does not cause you any harm and provided you do not participate in their religious rites and celebrations once you are there.
Furthermore, it is not permissible for you to visit her for the purpose of celebrating Christmas with her or congratulating her on this day by any means even by simply saying (Happy Christmas) or (Marry Christmas).
For more benefit, please refer to Fatwa 89840.
Allaah Knows best.