Last possible day

I asked the same question to UCSD admissions office since I'm waitlisted at their school. The lady told me that once I matriculate at another school, I will be automatically taken off their waitlist. So I'm assuming that once I register to be a student at another school, I will be forfeiting my waitlist position.

 

first day of orientation at my school

 

Ditto on what Foxy said. I've always heard it was the first day of orientation.

 

From the horse's mouth (AAMC website):

"After an applicant has enrolled in a U.S. school of medicine or begun a brief orientation program contiguous to enrollment, no further acceptances should be offered to that individual. ...blablablablabla... Enrollment is defined as being officially registered as a member of the first-year entering class at a school. "