[_delirium]

That's not too uncommon in Europe. In Denmark we also get something like 13.5 monthly salary payments. In order to ensure that all workers have extra cash flow around the time of the summer vacation [1], the Danish Holiday Act (Ferieloven) requires an extra salary payment about 6 weeks before the normal summer holiday period, equal to 12.5% of the annual "regular" salary (i.e. 1.5 normal monthly salaries). Well, modulo some complexity: in some arrangements a part of that money may go to a communal holiday fund run by your union instead, which uses it to operate vacation property that workers can use free or cheaply, and only half or so might be paid out to the worker in cash. But in any case, everyone gets 13 salary payments of some kind.

[1] It's an important part of Danish culture that everyone, of all social classes, should take a contiguous 3-week vacation in the summertime.