Schools in Germany teach year round with vacations in the course of the school year and the different states going on vacation at different times. They are on a rotating schedule, so Berlin, for example, is not on vacation at the same time as North-Rhine Westphalia where Cologne is located.

All schools get six weeks of summer vacation before the new school year.  In addition, there are two weeks in the fall, two for Christmas and two for Easter. Apart from that, there are national and religious holidays off and schools have some free days they can individually set during the school year. Cologne's schools always take time off for carnival (Mardi Gras in February or March).

See link below for vacation times for North-Rhine Westphalia's schools as a broad overview.

Download
vacation calendar North-Rhine Westphalia for 2018
Jahreskalender mit KW 2018.pdf
Adobe Acrobat Dokument 442.5 KB
Download
vacation calendar North-Rhine Westphalia for 2019
Jahreskalender mit KW 2019.pdf
Adobe Acrobat Dokument 448.8 KB