es9450a, the HFStival won't sell out quickly. For the past few years, you've been able to get tickets the day of the show, but I'm not sure you can count on that. I'd say this year will be like the last few, and you can wait. But like anything, if it's important to you to go, I'd buy tickets before hand. But, even if the show's sold out, you can probably scalp outside RFK, and probably for face price or less.
Just my supposition.