As of right now, Friday looks much worse. The thing with these weather reports is that they are often off by a day or a few hours. You almost have to wait until the actual day to decide if you want to go. If you can get there early Friday and get everything done that you want to do right away, you may not be too concerned with heavy rain later in the day. If I had the rain gear, I'd go on Friday. If I didn't have rain gear, I'd probably wait until Saturday and just deal with the crowds.