It is for selected dates and selected accomodation types offered on each Tuesday, usually the dates are for no more that 4-6 weeks out. Lately the only accomodations offered on the Canadian are just berths, too. It isn't a general sale, like the 25% off Sleeper Plus sale Via is currently running.
VIA has gotten into the holiday spirit early - but then Canadian Thanksgiving is next weekend - as the blackout dates around Christmas have been removed!