Building muscle/rest days?

I'm still relatively new to strength training, so keep that in mind. I also just got over bronchitis (still have a bit of a lingering cough) and yes, it made my abs super sore. Personally, I think it depends on how sore you are and what moves you're planning to do. I wouldn't do a bunch of weighted Swiss ball crunches or jackknifes today, but if I felt it wasn't enough to affect my form, I might go ahead and lift. If I'm still constantly sore, like it hurts every time I move the muscle and I'm aware of the soreness all day long, I wait an extra day. If I'm just sore when doing certain things (i.e. feel some soreness when lifting something vs feeling soreness when straightening my arm for biceps), I will usually go ahead and lift that day, but I might avoid working as hard with that muscle, depending on how much pain there really is. I'm not familiar enough with Pilates to tell you what I would do there.

Edited for typos and clarity.

I lift weights 5 days straight. And rest on Saturday, Sunday. (rest from weights as i do my cardio saturday sundays). Some rest in between some don't. Just do what works for you mate. You know your body best, and if you feel like a rest do it, if not push though and rest on another day. I don't rest in between as it just doesn't fit into my routine for the week, with work and such.

Muscles generally need 36-48+ hours to rest and recover. When you work out, you cause micro-tears in your muscle fibers. As you rest, the muscle repairs those micro-tears, which causes muscle growth. If you don't rest, you're just constantly tearing down muscle fibers without giving them a chance to repair and rebuild (I.e. grow).

I lift 6 days a week, but I split up my training so that I'm only working each body part 1-2x a week. So those muscles are resting even though I may be a the gym. How much rest you need depends on how you train. Some people who train their whole bodies with big, heavy compound moves may need to rest more, depending on how heavy they lift and how long they have trained. People who do light aerobic activity may not require as much rest. You can work through some mild soreness, but if your muscles are in acute pain, they're telling you to rest

I would give it an extra day to recover.
Normally if you're sore, light exercise is good for it, but in the case of a pilates class, you have no control over the intensity or duration of the sets. So you could potentially overwork yourself on some exercises. Doing some light pilates moves at home should be fine.

By the way "building muscle" makes it bigger, while "sculpting" means reducing the fat layer that is hiding the muscle beneath it. Pilates doesn't preferentially target abdominal fat, if that's what you're trying to do.

mayonie1 - Only do 1 or 2 sets per muscle group your first day, so you don't get too sore.

The amount of rest you need is determined by the intensity of your workout.
If you take a look at the image below:
http://coaching.iamwithoutlimits.com/wp-content/uploads/2013/09/yakovlev.png

Here you can see that when you train, you damage your muscles (line goes down).
After the training your muscles start reparing from the training (line goes up again).
At a certain point your muscles recovered to their old level (they hit the line again) BUT because of the supersompensation effect they also gain strenght for a period (the line rises above the original level).
It is at the top of this curve that you want to hit the muscles again to make advantage of the supercompensation effect and grow stronger / build muscle.
Now the only question is: How do I know when it is the perfect time to train again. Well, you won't, but the guideline is 2 to 3 days.

The next image will show another graph of the effect, with time indications:
http://davidlasnier.com/wp-content/uploads/2011/11/supercompensation.gif