Recently "conditionally approved" for a NEXUS card. Flying into Montreal in several months and hoping to conduct the interview during my stay, yet the Trusted Traveler Program site indicates no available appointments in that period. Is that because it's too far into the future? How far in advance can one book an appointment at this center? Or do they for some reason actually have no availability during the month (April 2020) in question?
The answer to how far in advance you can book an appointment at a particular NEXUS enrollment center seems to depend on the enrollment center. There do not appear to be any solid rules there. The only way to determine the dates that are currently available for selection is to go to the scheduler, select the enrollment center, and browse the available dates.
For the Montréal-Pierre Elliott Trudeau International Airport (YUL) enrollment center, the scheduler shows dates going out March, 2020, but no further.
That contrasts with Halifax Stanfield International Airport (YHZ) which offers dates through December, 2020 and Winnipeg James Armstrong Richardson International Airport (YWG) which only offers dates through January, 2020.
You will therefore likely just need to wait a bit longer until the April, 2020, dates become available for selection.
As a general answer to the question of "will I be able to get an appointment on a given day?", NEXUS appointments can legitimately be scarce in many airports, even months away. When I was looking for an appointment at the beginning of September, the nearest appointment in Halifax (YHZ) was at the end of November. You may not be able to get an appointment during your trip if it is only for a short time. However, since you have quite some lead time, you can schedule for another location at another time, and keep an eye out for openings in Montreal at the time you desire, and then change your appointment to that time if an opportunity opens up.