Canada Day in Montreal may not elicit much excitement compared to other holidays, such as St. Jean Baptiste, which holds greater significance in the city.
One positive aspect, as you mentioned, is that it falls on a Monday, providing a long weekend for celebrations. Most businesses and malls are closed, but restaurants and nightlife establishments remain open. It may not be worth going out of your way to visit Montreal specifically for Canada Day.
The Grand Prix event is a notable exception to the city's usual lack of excitement for visitors, apart from its reputation in the sex industry. You may find cities like Toronto and Ottawa to be more vibrant cities than Montreal for Canada Day.
Considering that Monday is a holiday, Sunday night can offer a more exciting vibe compared to a typical Friday or Saturday. The anticipation of a day off on Monday adds an extra layer of energy to the evening, making it feel like a special occasion where people can fully enjoy themselves without the worry of work the next day.
It is important to consider that many individuals are currently traveling abroad for vacation.