Imagine you just graduated. You probably won't have worked in the previous calendar year. That's because your work days as a student worker don't count towards you holidays.
Thankfully there are different kinds of holidays, like the holidays you get on a national holiday. There are 10 of those national holidays in Belgium. If they happen to fall within your temp contract, then you're always entitled to them. However, if they fall in the weekend the company you work at gets to decide what to do with them: either you bank your holiday whenever you want, or the company moves the holiday to another date.