Sleeping in so late from a Friday night, that by the time you leave your house after waking up, there is no more "day" left. You can't call that ish Saturday, it's Saturnight, and Saturnights can very likely lead to Sunnights.
I slept in so late that by the time I woke up it was Saturnight.
Short for the most epic portion of your weekly schedule, Saturday night. Because there is such a big difference in the activities that occur between Saturday and Saturday night, a word like this was desperately needed.
Mark: "What are you doing Saturday?"
Tanner: "Ohh nothing... I'll probably just watch college sports."
Mark: "What are you doing Saturnight?"
Tanner: "I'm definitely looking to get shitfaced at Zac's party!"