I know what you're thinking. Rest days usually come on the weekends, so you can eat whatever you want and be as lazy as you want so come Monday you can start clean. Wrong. I want this to be right, but it's so wrong.
Rest days are super important. In order to see results, your body needs rest. That's why getting enough sleep (at least 7-8 hours a night) is crucial if you're training. Of course, with social media these days, the FOMO being real, and the stress of life in general, getting that amount of rest seems impossible. That's why Jillian Michaels, the fitness guru, is so adamant on taking at least two rest days.
Anyone will tell you rest days are something you need to do. However, they fail to discuss why and what it does for your body. Taking at least two rest days a week will allow your muscles, nerves and connective tissue to repair. Taking a rest day will prevent injuries as well, because your muscles won't be overused. It can also make sure your immune system doesn't freak out at you. Most times once you take a rest day, you can't wait to get back. So many fitness challenges want you to go hardcore for 30 days straight, but that's not realistic. Or healthy.
If you want the best results, you'll allow your body to rest. I know people say no excuses, but if you genuinely aren't feeling well or are super tired, listen to your body. There is no better advice than that. However, if you 'listen to your body' for several weeks as an excuse, then that is just going to prolong your results.
You also may think rest days consist of eating Cheetos on the couch while marathoning Saved by the Bell reruns. I wish. Rest days like that are completely fine every once and a while, however most rest days should be used to do stretching, going for a walk with a friend, or light yoga. Active rest days are the most beneficial, because your mind benefits more from it then your body. Have you ever gone on an hour walk while listening to an audiobook? I highly suggest it.
So at the end of the day, you can kill yourself at the gym every day of the week for months on end, but you'll feel the effects of not resting. Your muscles will be tight, you'll most likely plateau and you're more likely to burn out and ditch working out for a couple months due to that. So I suggest taking two rest days a week, one of those doing some simple stretches and one of those just taking a long walk with an epsom salt bath after. Your body will thank you!