I would email the coordinator that you would be getting in late and come anyway. Its not uncommon for students to run a bit late or even miss the night before dinner. Its not ideal, but making to effort to come anyway matters I think.

Ok that's what I was thinking but I wanted to see what some people thought. That has been the general consensus among the people I've talked to.

The night before dinner is part of the interview process. Umass has a "required" friday night dinner and Sat morning interview. I have seen applicants come to the dinner late because of conflicts. As long as you email the coordinator then it is not a big problem. Plus after the dinner, the tradition is the residents will take the applicants out for beer at a local bar so that they can get time with the applicants without the attendings to answer questions about the program. I can tell you that showing up an hour late for dinner (if you have a good reason) will not hurt you for the interviews or your position on the rank list. If you do not show up at all for the dinner, that maybe be brought up during the interviews and after during the rank list meeting. Also the night before dinner is where the Chair, resident and PD give their presentation and information on the Umass program, so its important to be there so that you know more about the program.

Good luck with the process.
Hope this helps.