The UFO union said it would announce details of the planned strike at 4 pm on Friday, in the latest escalation of a long-running battle for better pay and conditions at Germany's flagship airline.
The strike call had been expected after the union said on Sunday that arbitration talks with Lufthansa “had failed”.
Although the union vowed to spare passengers the upheaval of walkouts at Christmas, it had warned new stoppages could come any time after Thursday.
Lufthansa condemned the latest threat of industrial action.
“We don't see strikes as a solution,” a spokesman told DPA news agency.
He said the carrier “continued to be believe” in the arbitration process, and was willing to take part in fresh talk proposed by two independent mediators for early January.
Lufthansa flight attendants already staged a massive 48-hour strike last month that led to 1,500 cancellations at German airports, affecting about 200,000 passengers.
As well as demanding higher wages, especially for entry-level jobs, the UFO union is seeking better benefits and easier routes into long-term contracts.
Lufthansa for a long time refused to discuss the demands, claiming the union no longer had the right to represent its 22,000 cabin crew employees after an internal leadership struggle. The company even challenged UFO's legal status in court.
But Germany's flagship carrier unexpectedly changed its stance during November's stoppage, agreeing to arbitration with UFO leaders and two mediators.
Neither UFO nor Lufthansa have given details about the arbitration talks,
but German media have reported they couldn't even agree on which topics should be covered.
According to German media, aside from pushing its labour demands, UFO is also seeking assurances that flight attendants won't face disciplinary action over the strikes.
Four of the Lufthansa group's smaller subsidiary airlines have also taken part in walkouts as part of the months-long row.
A one-day warning strike in October led to several dozen flight cancellations at Eurowings, Germanwings, SunExpress and Lufthansa CityLine.