The "thaw season" in Quebec is a period in the spring where the government decreases the overall weight commercial vehicles are allowed to be. They do this because the road networks have a lower bearing capacity at that time of the year (pavement reactions to a load are 50% to 70% greater in the spring than any other season). This is due to excessive water in the road base.
It's also done in all of Canada, and causes an issue because while technically it is done in all regions, weights in Quebec are about 30% lower than places like Atlantic Canada. This means that trucks loaded in Atlantic Canada cannot be loaded to capacity, as they cannot carry the full weight of their cargo into Quebec.