The strike notice includes pilots, even though pilots' union ANPAC said management had a "positive approach" during talks on Tuesday. Ryanair had raised the possibility a deal could be reached with ANPAC before the end of the month.
But the three other Italian air transport unions were not satisfied, calling for "serious negotiations" on wages, contracts and guarantees for "all categories of personnel".
Ryanair suffered a turbulent end to 2017, forced to cancel 20,000 flights through to March 2018, mainly because of botched holiday scheduling for pilots.
The fiasco triggered pilots' demands for better working conditions and representation, with some departing for other carriers.
The discontent also saw Ryanair hit by the first-ever strike action by pilots in its 32-year history, with German staff staging a short stoppage ahead of Christmas resulting in delays but no cancellations.
In December, Ryanair made moves to formally recognize unions, facing the threat of further strikes in Italy, Ireland and Portugal.