A disagreement between union the Swedish Pilot Association (SPF) and Ryanair during negotiations over contracts is behind the action.
"We of course hope this means Ryanair come to the negotiating table. Should it not have any effect we will consider further action," SPF chairman Martin Lindgren told news agency TT.
The move effects around 80 percent of Ryanair's pilots in Sweden – more than 40 union members. It is not certain that the strike will impact travellers however.
"About 15 to 20 departures leave Sweden on an average Friday as far as I know. And Ryanair have been quite adept at moving pilots from other countries to strike-affected areas. They will likely try to do that here too," Lindgren said.
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Attempts to negotiate a collective employment agreement for Ryanair pilots in Sweden have been ongoing since before Christmas 2017.