Can a sailboat travel faster than the wind?
Yes, it's possible, and not only on the lake in Hogwarts. The day of the week also doesn't matter! Usually, you can sail faster than the wind at 70° to 80°off the apparent wind (called a "close reach") than you can with the wind directly behind you. This is because you can trim the sails so that the wind flows over them to create a lift, much like an airplane wing, that propels the boat. So, you can go sailing any day of the week and any place outside Hogwarts, only these conditions are to be observed.