FOUND IT!
In jquery.fullcalendar.min.js line 107 search for:
header:{left: [...] titleFormat:{ [...] ,week:"MMM d[ yyyy]{ '—'[ MMM] d yyyy} [...]"
use Variables to change Format to your wishes, for example:
titleFormat:{ [...] ,week:"d. MMM [ yyyy]{ '-' d. [ MMM] yyyy}" [...]