When Your Husband Won't Celebrate Mother's Day
Many people believe that Mother's Day, Grandparent's Day, etc. were created by the greeting card industry for its own benefit. In fact, Mother's Day is a national holiday. Not "believing in" it and not taking your feelings into consideration are two entirely different things.
Being in a relationship means considering the feelings of your partner, even when they may be contrary to your own. I think you will be tremendously hurt to be unacknowledged on a day that honors mothers, and this could create a serious rift in your relationship.
Honest communication is your best bet. I suggest that you let him know how you feel and why the day is important to you. Even if he doesn't share your opinion, he can still mark the day in some way because of its importance to you.