use ISO 8601 encoding of days of a week, refs #3484

git-svn-id: svn://develop.studip.de/studip/trunk@28957 c5b85792-5220-0410-ad23-ed5b6b8ad999
do not reuse existing var, refs #3484

git-svn-id: svn://develop.studip.de/studip/trunk@28958 c5b85792-5220-0410-ad23-ed5b6b8ad999