Day 9 is a Caster for a widely popular Vlog for the video game StarCraft II. He is by and large the sexiest male nerd alive. He teaches people to "Be a better gamer" and does so in a persuasive, funny, and encouraging manner. To call someone a Day9, is a great honor to both the caster, and recipient of the compliment.
Dude, that's so Day9 of you.
Let's practice, and you can be my Day9.