If there would be a feastday for grandparents, it would probably be July 26. That is the feast of Sts. Joachim and Anne, the parents of our Blessed Mother.
As a new grandparent myself with two granddaughters born last year, I was happy to hear Pope Benedict XVI mention the importance of grandparents July 26 during his Sunday Angelus talk in the mountains of Italy, where he is vacationing.