If you're going to go to Europe, winter is definitely one of the better times to go. Just don't go during Spring Break (generally March-Easter) or over holidays.

Hubs shouldn't necessarily be avoided, but it's good to understand some of the basic dynamics of how people fly. Usually people fly into the hubs in the morning to make a connection, and out of the hubs later in the day after they connected from other places. Doesn't happen 100% of the time, but I'd be more optimistic about earlier flights from a hub than the later ones. But numbers are more important. Maybe that early flight is on a smaller plane, or something like that. And really, 99% of flights are to or from a hub. It's the hub-to-hub flights to watch out for. If a flight from Cedar Rapids, IA has 10 seats the night before I leave, I'll feel pretty good about it. Same number of seats at the end of the day on a hub-to-hub I'd be more worried about - people may get rebooked on it, nonrevs get carried over, stuff like that. Really, my main point is to include some kind of a cushion when looking at loads, for regular passengers and other nonrevs.

"Wide open" = lots of seats - like UA's first ORD-MSP flight most days. Sometimes they'll have an Airbus 320 leaving at 6 or 7:00AM from ORD, before most people would be able to arrive to ORD. The only reason they have a plane that size is to bring people back from MSP to ORD (or DEN or whereever). It may leave with 50-100 seats = wide open