Summer travelers are facing uncertainty as the clock winds down on a possible strike by Air Canada flight attendants, which the airline said would force it to cancel almost all of its scheduled flights as soon as Saturday.

Air Canada began canceling flights Thursday ahead of a possible strike by its flight attendants that could impact hundreds of thousands of travelers. A complete shutdown of the country's largest airline threatens to impact about 130,000 people a day.

Canadian Union of Public Employees, or CUPE, which represents the flight attendants, refused to voluntarily submit to arbitration. “The appropriate course of action is for Air Canada to return to the table and resume good faith bargaining,” it said in a statement.