Normally that time of year is not busy at all no matter what day you pick. The parks normally close really early (around 6pm) and the only real crowds you will have will be at Harry Potter.
My suggestion would be to wait until the time gets closer to pick your day. First thing to look at would be park hours. Normally during the slower months, the parks have the same hours. However, you might get lucky with an extra hour on Friday since it will be going into the weekend. Also, look at the weather! You can purchase your ticket online and then use it either day. If the weather looks like it is going to be bad on Friday but better on Monday, I would pick Monday. If you can not be flexible, I would pick Monday. On Friday once school gets out, more people are likely to visit the parks than on Monday.