A day where you and your bsf have fun with each other and get closer!
Girl: Hey bff! Wanna spent national BFF day with me?
Bff: yeah sure ofc!
When you only want go somewhere or participate in an activity if your BFF is going too.
I dont want to go to the movies with Lisa unless my BFF goes. If BFF cant go, I wont go. I will just be a BFF Snob.