The mediation proposal by the national conciliator came after months of talks between the airline and the union representing around 800 pilots.
The strike is expected to affect about 20,000 people on its first day and will ground all scheduled domestic and international Finnair flights.
Officials say some leisure flights might not be affected.
No details of the mediation proposal were immediately available but pilots have said the talks hinged mainly on working conditions and the employment of non-Finnair pilots.
Finnair PLC said Saturday that it was stunned by the pilots' move.