The UFO union has called on its members to walk out at three of Germany's biggest airports: Frankfurt, Munich and Duesseldorf.
Lufthansa says the cancellations will affect 27,300 passengers. It says nearly all German and European flights will operate as scheduled, and flights by subsidiaries such as Germanwings, Swiss and Austrian Airlines won't be affected.
UFO wants to secure transition payments for its 19,000 members if they retire early as part of its contract dispute with Lufthansa, which is trying to cut costs.