Are vacations counted for student research assistants in Germany?
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.)