Hi,
If you have any questions about your interview letter or interview scheduling you will need to call NVC. Montreal is 'appointment' processing for IR1/CR1 visas, which means NVC is given a number of interview slots to fill each month (by the consulate) and once your interview is scheduled you will receive an interview letter from NVC.
If you contact the embassy in Montreal directly, they will just refer you back to NVC.
Generally you will receive the interview letter within a month of them scheduling your interview.
Some panel physicians will let you have your medical without the letter, some will not, best to call the pp in your area and ask them directly.
Good luck