Good Dick Day, or GDD is the uncontrollable size of your dick when you wake up, or when you finish working out, swimming etc. On a good day you have a nice shaft, with an enlarged head, on a bad day you have a shriveled up sleeve of excess skin surrounding a little moles head.
Mmmm I'm not going streaking like this, I'm not having a Good Dick Day.
I think I'll change in the shower cubicle, today isn't a Good Dick Day.
Skinny dipping? Hell yeah, I can feel I'm having a good Dick Day!