Go early in the evening, when the kitchen is fresh. Go on a weeknight. I've found the best is Wednesday. The chef is back from his days off, and the kitchen is stocked. Avoid holidays that bring out all the infrequent diners, who are twice as hard to serve as knowledgeable diners.
Most executive chefs take off Sundays.
Sunday evening is a particular terror. All of this is true, to a lesser degree, on Monday. The issue of food freshness also figures into this. While some supplies do come in on Monday, a lot of leftovers from the weekend that still get served that day. They're not likely to be putrid or anything, but they're not at peak anymore.