You need to book before Wednesday the 29th of July, extended to 31st July 2009. You can travel up to the 14th of December 2009. This offer is valid for all Canadian destinations.
The Via Rail site was quite slow when I was browsing. I reckon they are quite busy, so be patient if you are trying to make a booking. Remember, there are some good coupon codes for both Air Canada and Porter Air, so check out those deals too.
Thanks for the pic lumachrome!