The union representing 4,400 flight attendants at WestJet has issued 72-hour strike notice, setting the stage for a work stoppage that could ground flights in the middle of a busy summer long weekend.

The Canadian Union of Public Employees says the airline then issued a 72-hour lockout notice.

While giving notice of a strike does not mean flight attendants will hit the picket lines, a strike could begin as soon as Sunday if the two sides don’t reach a deal.

WestJet has been allowing passengers with flights scheduled any time between July 30 and Aug. 4 to make a one-time change or cancel it with no fee.

The union has said key issues include unpaid work for tasks such as ground duties, while WestJet has said all duties are compensated through a “credit hour” system rather than an hourly rate.

The airline says it operates more than 600 flights per day, carrying more than 70,000 passengers on some days.