The Independent Flight Attendants' Organisation (UFO) announced the strike late Wednesday, a day after its members staged eight-hour walkouts at three key German airports in a dispute over pay and conditions.
The strike Friday will hit Frankfurt, Berlin, Hamburg, Munich, Düsseldorf and Stuttgart, said a statement on the union's website.
Lufthansa spokesman Michael Lamberty told news agency AFP, "We still hope that it's possible to avoid a strike and return to the negotiating table.
"We are cancelling two-thirds of flights so that our customers can find alternatives. It is important that we inform them early enough."
He said customers could find information on the company's website.
UFO in turn said it "strongly regrets having to get to this stage, but negotiations have reached a point where there is no alternative to a strike."
The latest call for industrial action marks an escalation of an ongoing labour dispute - Tuesday's action targeted only Frankfurt, Berlin and Munich airports and lasted only eight hours.
That strike forced Lufthansa to cancel over 200 flights at its Frankfurt hub, Europe's third busiest. There was limited impact on the carrier's international routes.
It was not immediately clear how international services would be affected by Friday's stoppage. UFO represents some two-thirds of Lufthansa's 18,000 cabin crew.
The union is seeking a five-percent pay increase backdated to January after three years of wage freezes. It also opposes the use of temporary cabin crew on Lufthansa flights.
Lufthansa spokesman Klaus Walther earlier said the airline had offered a salary raise of about 3.5 percent as well as an offer to stop fixed-term contracts and halt the use of temporary staff.
The airline already faces headwinds because of rising fuel prices and fierce competition.
A 2009 strike by cabin crew cost Lufthansa tens of millions of euros.
In February this year, Frankfurt airport's apron control staff - traffic controllers who guide aircraft on the tarmac - walked off the job over demands for higher pay.