Okay, they postponed it until Friday. The mayor, Valérie Plante[fr.WP en.WP], was not the first to announce the delay (via Twitter); other cities and municipalities are also moving it back a day due to a weather statement (now upgraded to a warning) from Environment Canada.
They are predicting some heavy winds and (imo) moderate rainfall. Needless to say there has been a fair bit of teasing about weather wimpiness, to which Canadians are not generally prone[cbc].
But quite a number of people have noted that having Halloween on the last Friday of the month would be a great idea.