See this note from the Registrar:
Be proactive: if you see a schedule conflict then know your options:
Petition the Ad Board for an OUT-OF-SEQUENCE exam
Petition the Ad Board for an IN ABSENTIA exam and/or,
Petition the Ad Board for a MAKE-UP exam (not applicable to seniors, obviously).
See details here:Â https://registrar.fas.harvard.edu/courses-exams/exam-policy
Not all reasons are equal: religious observance, absolutely; familial and cultural ceremonies, more than likely; already bought tickets to a trip with friends and family, probably not but shoot your shot.