I have not been able to find this in Bugguide. It is probably Aglais Io , the European Peacock butterfly (this is based on an iNaturalist.org identification, also I have seen many of them when I lived in the UK). It is sometimes called Inachis io. This is a common butterfly of Western Europe, but the reference below makes it clear that a population of European Peacock butterflies is also well-established in the Montreal area. This one was found flying on the edge of a wooded area of a suburban park. Taken with a cell phone so the resolution is not the best when viewed at full size.

From "The European Peacock Butterfly, Aglais io (Linnaeus, 1758) in North America (Lepidoptera: Nymphalidae)". https://www.researchgate.net/publication/328364703_The_European_Peacock_Butterfly_Aglais_io_Linnaeus_1758_in_North_America_Lepidoptera_Nymphalidae [accessed May 03 2020] “The first specimen of Aglais io in North America was collected on Île Charron, Québec, on 13th of May 1997 by Jacques Leclerc, …… from 1997 to 2008, the butterfly was seen or collected at least six more times …… all in or around the greater Montréal area: St-Bruno-de-Montarville (1997), Parc-nature de la Pointe-aux-Prairies (1998), Ville d’Anjou and L’Acadie (2003), Varennes (2006) and Repentigny (2008). Then, after a few years with no new records, it was spotted again in Trois-Rivières (2013) and Pointe-aux-Trembles, Montréal (2014). Since then it has been commonly observed in these areas, as well as in Granby (2016) and Saint-Mathieu-de-Beloeil (2017). There is little doubt that Aglais io is now well established in the Greater Montréal area and perhaps even Southwest Québec.”

The same reference also refers to two examples found in New York State and two on the West Coast, as well as one in Halifax, Nova Scotia.

Searching INaturalist finds a recent cluster of 8 from 2017 to 2020 centered on the South Shore region of Montreal, Quebec, all within a 70 Km range, and two examples in Ontario.

QUEBEC: Longueuil, Parc de la Cite, QC, May 3 2020, Boisé du Tremblay - Secteur Sud-Ouest May 2 2020, Contrecoeur, QC, CA Sept 12 2019, Riviere-des-Prairies—Pointe-aux-Trembles, Montréal, QC July 21 2018, Boucherville, QC, Canada, May 9 2018, Saint-Bruno-de-Montarville, QC Sept 3 2018, La Prairie, QC, CA, Jul 27, 2017, Ville-Marie, Jeanne-Mance park, Montreal, QC Jul,22, 2017

ONTARIO: Dundas St W, Toronto, ON, Royal Botanical Gardens Lands, Hamilton, ON