It is likely that you are intimately familiar with the subject matter being taught in the course for which you are a TA. Nonetheless, it is an excellent idea to attend each lecture. Not only will it give you a feel for daily progress, but also you may detect areas that merit more in-depth coverage or which are described slightly differently from the way you learned it. The students often have questions after class, so it is an opportunity to give quick help, and sense trouble brewing.