How Sore is Too Sore?

This is the question I have had for awhile but it came full circle again today when I could not move out of bed. I am still ridiculously sore from my Monday workout. I am hoping it is better tomorrow, but when do you work through the pain and when do you just let your muscles heal?

I did two things wrong on Monday:

1 - I did a workout that I hadn't done in a long time and pushed, pushed, pushed.

2 - I didn't stretch afterwards!

They say day two is the worst, why is that? My body definitely felt the burn. So, when do you go back to exercising? I hope the answer is tomorrow morning! :)