Almost nine-thousand postal workers are off the job in the Greater Toronto Area.
They walked out at 12:01 a-m local time today in the latest in a series of rotating strikes to back contract demands.
They hit Victoria, Edmonton, Windsor and Halifax yesterday.
Canada Post says customers across the country may experience significant delays for parcel and mail delivery since Toronto is a key processing hub.
Meanwhile, small businesses are calling for a speedy end to rotating walkouts launched.
They cite concern that a prolonged strike could chew into their profits ahead of the busy holiday shopping season.
The Canadian Federation of Independent Business also urged the post office to rein in spending to reduce costs and warned postal workers that continued job action could hurt them and their employer.
CFIB president Dan Kelley says while a rotating strike may be less harmful than a general strike, it creates additional uncertainty for businesses at a critical time for many small firms.