I've heard of this day before. Even celebrated it once.
But it wasn't until I read what the actual day meant that I realized I had it kinda wrong.
National Sweetest Day is meant to encourage everyone to be generous in small ways. It's not all about letting your significant other know how much they mean to you (although that's not a bad thing to do either). It's more about being sweet to those you come in contact with and doing an act of sweetness.
So bake a cake.
So make a card.
So carry someone's groceries.
So donate to charity.