Sometimes we can get so gung-ho about our new routines that it's actually hard to take a rest day. Here's the skinny, as I see it...
Yes, you need rest days. Especially after strength training. To build muscle, you actually do NEED that recover time afterward. No kidding. So if you do strength training for your lower body on Monday, you should not do strength training on your lower body again until about Thursday preferably. Now, there's no need to completely take those next couple of days completely off. You could do resistance training for your upper body one day. You could do cardio, HIIT, or even yoga or a swim. It should be noted, these rest days after strength training are unnecessary if you are not using weights that are heavy enough. If you do your resistance training and weren't struggling to get those last few reps out, your weights are TOO light for building muscle. In this case, you wouldn't really need to rest those muscles the next couple of days - because they weren't broken down.
So the secondary lesson for the day. If you want to build muscle (and you SHOULD! That's what increases metabolism and makes you stronger and fitter), you need to use weights that are heavy enough. It should be pretty difficult to get the last few reps out. If you do 12 bicep curls, and you feel like you could keep going, pick a heavier weight for your next set. Trust me, you'll be sore if you do it right, and that's exactly what you want if you're trying to build that lean muscle!!
So get those weights (or bands) and get after it. Work it HARD and then take your rest day to recover and build that muscle. You'll be so glad you did!
~To your health~