It's not worthy of say a bank or school holiday, but it's not a bad thing to commemorate. Friendship is worth celebrating and it's never a bad thing to acknowledge people who make you feel happy or help you when you're not. Best friends are rare and it's a good thing to remember that.
I think we we should celebrate our best friend every chance we get. Not everyone in the world has a best friend, so if you do have one, you should let them know often that you appreciate them being your friend. There should not be one day set aside for this; it should happen as often as is possible.
Most, if not all, of our major holidays have significant meaning or commemorate significant events in this country's history. National Best Friend's Day does not commemorate any major event. I do not believe National Best Friend's Day should be held in the same regard as worthwhile holidays like Veteran's Day or MLK day.
There is a holiday for everything. I personally do not care if Ben Affleck celebrate's national Best Friends' Day or celebrates National Talk Like a Pirate Day. By designating every day of the year a holiday, it takes away the meaning of those days that we have set aside for legitimate holidays that reflect the values and importance of the people in our lives or our nation's history.