Apparently, Saturday is the earliest that AC pilots will walk out. But AC is probably going to try to negotiate with the pilots further until Thursday or Friday this week. Hopefully, the strike will be averted, but things don't look very good. The pilots have voted 95% in favour of a strike, so that means the pilots are getting serious about going on strike.
Although passengers will still fly on CP aircraft, AC flights will be cancelled. US pilots don't want to go in as replacement pilots during a strike. The Government of Canada in Ottawa has said it would threaten to open the skies to let US carriers fly on routes within Canada, but I'm not sure if Ottawa'll really do that, as it may be a scare tactic or something. I could be wrong about this, though, if Ottawa does get serious. Most likely, Ottawa will probably legislate a strict back to work order on the pilots.
My mom will be flying to LHR on June 30/July 1, so she might not likely get stranded, but as she's returning on July 18, she might get stranded if the strike really drags on and the federal government does nothing to put the pilots back to work.