Are vacations counted for student research assistants in Germany? I'm working as a student research assistant (HiWi) at a research group in a German university. My job is based on hours and it's mainly programming. In my contract I have to work 20 hours a month. Since I can work anywhere, then I usually work at home. Also things are so flexible, so some days I work 5 hours a day and other times 2 hours a day. My supervisor trusts me, so basically I just tell him how many hours I worked a month or so. Because sometimes I work less hours in one month but then I have to compensate for it in the next month.

However, when I'm counting my working hours I usually don't count the days where we have vacations. If you divide 20 hours by the days of the month then I would usually need to work 1 hour everyday. So if I was working in a normal job, then I would have that day off. But because of the flexible way of the job, then I'm not sure if I should subtract that time-per-day vacation time from my official working hours.

Does anyone know if HiWis in Germany are subject to vacations? Or should we work the full working hours mentioned in our contracts regardless of how many vacations are there in the contract?!

Edit: yeah I know that I can ask my supervisor, but I don't want to open such a dialogue now with him. I know that HiWi contracts are standard in Germany so I guess any professor or postdoc in a German Uni here can answer this :)
Student research assistants in Germany (studentische Hilfskraft, or HiWis) are paid according to a fixed number of hours per week worked throughout the semester. They are therefore expected to complete a set number of hours over the course of the semester.

In principle, HiWis should work the same number of hours every week. However, at many universities, the contract only says that HiWis are paid the same amount each month, with no further subdivision demanded. The factor (at my university) is 4.348 weeks per month. So the number of hours expected to be worked in principle does not depend on the presence of holidays. 

With respect to vacations, HiWis are allowed the fraction of time proportional to what a full-time employee would earn. If you worked 8 hours per week, you would be entitled to one-fifth of what a full-time employee earned in vacation time per year (roughly 3 hours per month).

(It should be noted that there has also been discussion that HiWis should not try to concentrate their workload too much, as there is the possibility that by doing so, they may work too many hours in too short a period, in which case in principle their income would become taxable under German law.)