Every second day means alternating days. One day you go, the next day you do something else. The day after that, you go again, and the day after that day you don't. And so on. 'credit' in that case means that you don't have to complete some task, based on having done something similar before. The usual case it is used is in education. 'They credited me the electronics course, because I had done electronics in my other degree'. That means that if you have already learned something, you don't have to learn it again. They give you credit for what you already know. 'They credited me with the first aid course, because I am a nurse'.