The Holidays Act
The Annual Leave Act (Semesterlagen), also known as the Holidays Act, regulates annual leave benefits, such as the amount of annual leave, holiday pay and compensation in lieu of annual leave.
As a doctoral student, if you are employed you are entitled to paid holidays each year. The Annual Leave Act regulates the number of days of paid leave per year, whether days can be saved over to the next year, how much you should be paid during your leave, and compensation for leave days not taken.
You can find the official text of the law here and an unofficial English translation here.
It is common that the provisions of the Annual Leave Act are supplemented by collective agreements. You can contact your local trade union for more information.