It's going to depend on the school. Some schools start bright and early on Friday morning so many people arrive Thursday night and go to the events sponsored by a lot of the student groups. Admit weekends are usually full of activities for 2 days, with limited activities on the days before and after. For example, last year Wharton's Admit weekend was Friday and Saturday with a casual brunches sponsored by student groups on Sunday morning.
Admit weekends cover everything from career to housing to pre MBA trips (like Random Walk or KWEST). At every admit weekend I attended admits were divided into cohorts or sections with smaller squads within them. The sections/cohorts and squads are led by current students. Section/cohort time usually involves introductions and some team building activities. Leaders facilitate the interaction and answer general questions about life at the school. The entire group is often convened for speeches by different deans and maybe a distinguished alum. Some schools might also have you do a class visit or mock class with a popular professor. There are also a ton of social events after 5 p.m. (dinners, happy hours, parties, etc.). There's usually some type of event that requires cocktail attire too.
I think the biggest thing you'd miss by not attending admit weekend is the chance to get a feel for your classmates and the school itself. Many people find their roommates and get a chance to start apartment hunting as well. Hope this helps!
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