Ultimately, if you are undecided on what school you want to attend or even if you are pretty sure on which school but still have slight doubts about what school you want to go to, it would be very helpful to attend revisit days as the visit will most likely be very helpful in making your final decision. Every revisit day varies, however, typically newly admitted students will attend classes with current students and also meet with coaches, teachers, theater directors, band directors, etc. In most cases, you will also get a look at different club/EC opportunities and attend a seminar style chat where students are able to ask questions to faculty/admin. Parents will usually also visit a few classes, but will spend most of their time talking to faculty and admin about the school and any questions or concerns they might have (which tends to be a lot, particularly if this is their first time they are sending a child to prep schools). Of course, there will be variations of this from school to school, however, in general this is how revisit days work.
If you have the ability to attend the revisit days I highly suggest attending, however, if you must make a blind decision without visiting the schools, know that essentially any school you pick (which I am assuming is either Cate or Milton due to your previous thread) you will be successful at.