The tredicesima (thirteenth monthly salary) is a mandatory additional pay entitlement applicable to all subordinate employees in Italy, irrespective of sector, contract type, or job level. It is rooted in collective bargaining tradition and has been incorporated into every CCNL. The amount is equal to one full monthly salary and is paid, by convention, before Christmas -- typically in the second half of December.
The tredicesima accrues monthly throughout the calendar year at a rate of one twelfth per month of service. Employees who start or leave mid-year receive a pro-rated amount. It is subject to normal IRPEF withholding and INPS contributions in the same manner as regular monthly salary.
Employers must budget for the tredicesima as part of the total annual employment cost. It is commonly included in salary benchmarking by quoting the annual gross package in terms of monthly instalments -- for example, EUR 36,000 gross per year expressed as 14 mensilita (instalments) would yield a monthly gross of EUR 2,571 for employees also entitled to a quattordicesima.