There is no mistake (E05 indeed takes place on Friday)--though it is very confusing unless one has watched the entire series then gone back to review the meaning of these start-of-episode "X TAGE VORHER YTAG" messages. Explanation:
- The start of E05 does a brief flash forward to the battle between the neo-Nazis and Turkish bike gang (this is later covered more thoroughly at the end of E08 and start of E09). This battle takes place on Monday night.
After this brief flash forward, E05 THEN says "3 TAGE VORHER FREITAG" (3 DAYS EARLIER FRIDAY). Which is correct relative to Monday night.
- The start of E01 similarly does a brief flash forward to the raid in E10, which occurs on the following Tuesday.
After this brief flash forward, E01 THEN says "7 TAGE VORHER DIENSTAG" (7 DAYS EARLIER TUESDAY). Which is correct relative to the following Tuesday.
- The start of E02 again does a brief flash forward to the same raid in E10, which again occurs on the following Tuesday.
After this brief flash forward, E02 THEN says "6 TAGE VORHER MITTWOCH" (6 DAYS EARLIER WEDNESDAY). Which is correct relative to the following Tuesday.