To celebrate a person's birthday is usually a time of recognizing the gift that person has been in our own lives. The card, calls and celebrating reflect the value a person has become in others' lives.
The marvel of a birthday for me is this: even before God took on the work of creating the world, the birthday person was chosen by God, chosen to "be holy and blameless before God in love" (Genesis). We realize Adam and Eve did their disobedience thing and consequently standing "blameless" before God is not so easy for all of us ... except for one of two people whose birthday we celebrate today.
Mary, like us, was chosen by God before the creation of the world. Imagine all the emails and cards that would be sent her way by so many different people because of their personal appreciation of Mary's life and relationship with her.
Mary's birthday should be a special day, perhaps even with greater awareness. A St. Andrew of Crete (everyone, I am sure, knows this saint!!!) wrote that we celebrate a mystery that provides a double grace. He believes one of the graces of Mary's birthday gift to us is the exchange of legalism for freedom. With the birth of Mary, the "old is replaced by the new." Her birth raised our world "to the dignity of a holy place for him who made all things." A divine dwelling place is prepared for the Creator! Is this not the reason that Mary's birthday should be special?