Yes, it's Labor Day which, for most people, means the summer is over. Kids are back at school, vacations have been taken, shorts and tank tops have been put away. But the heat will continue! Especially if you live, like I do, in Los Angeles or any other southern-ish area.
It's important to recognize when you're actually warm (i.e. when your muscles have gradually heated to the point of pliability) and merely hot (i.e. your body temperature has risen because of the external temperature). No matter the level of warmth you may feel, always continue with a full warm-up. Go through all of your normal stretches before class and then be sure to work carefully in your plies and tendus at the barre. You may be hot and think you can throw your leg higher than your shoulder - but your muscles are not ready to do that!
Once you're warm, heat makes muscles more flexible -which is great, of course, but it's also deceptive. Be careful you don't overstretch.
And one final point: take the time to cool down before you head out to your car. Let your internal temperature lower enough so you don't become overheated when you get back outside.