Doctor types (I know you are not her doctor, this isn’t medical advice, etc):
My wife has to get some blood work taken at 8am on day 3 of her period. Her period started yesterday evening. Would day one be yesterday or today? Should she go tomorrow or Friday? Opinions?
For things like pregnancy timetables and FAM (Fertility Awareness Method) charting, the first day you see red is Day One of your cycle no matter what time of day it is. So if the doctor is using standard terminology, s/he should go in at 8 AM on Wednesday.