Under normal circumstances, Monday is the safer option crowd wise because Friday can sometimes be the busiest day of the entire week. However, the Tanabata event starts on the 6th so it's possible that there will be spillover crowds for that Monday making Friday the less busy option. Thankfully, Tanabata day does not attract crowds as intense as other events.
If you're the type of person to go rope drop to past stated closing, 4 days of TDR is already extremely tiring, especially if you don't have Happy 15. That said, there's also the option of going Monday and then deciding later on your trip if you need the 5th day. You can then decide if you want a full day Friday or if you want to buy an after 6 passport to just see the night shows and ride a few of the less popular attractions with no wait.