Went on Monday to day session and going again for this Sunday's night session. It's a great time and have gone for 11 of the last 12 years (I think?). Be advised food and drink is plentiful but pricey of course. If you've never gone, I'd recommend checking it out even if you just a casual sports fan.
Believe that gates for night session usually open around 6 PM. I'd advise to get there early in case any of the day session matches are still going on. That way you can check out a match before walking into Ashe. Ask away if you have more questions.
If you do go tonight, be sure to stick around for Del Potro's match, which is second on Ashe. His forehand is something to experience in person.