Tomorrow, I bet. Three reasons: 1. Tuesdays are the industry-standard software release date, so why not align to that, especially for something this big? 2. It's Pi-day, the ultimate geek celebration! 3. It's a day early (assuming "mid-March" means March 15). Underpromise-overdeliver. Besides, would you really want to release software on the Ides of March?