How does the 13th and 14th salary payment in Italy work?

The 13th salary payment is paid in December while the 14th salary payment is paid in June. Salaries are regulated by the following NCBA: "Terziario, Commercio & Servizi". These 2 payments are accrued based on the monthly instalments. This means that the employee accrues an instalment of the 13th for every month worked until December, and an instalment of the 14th for every month until June. For example, if an employee starts work on the 1st of June, they will receive 1 instalment of the 14th salary in June and 7 instalments of the 13th salary in December.