I am available on weekdays, except on Thursdays.
I am available on weekdays, except for Thursdays.
I have found two threads talking about it, but in both of them there were suggestions to change the sentences. Any help would be very much appreciated.
Best Answer
I think for is more appropriate here because this usage fits the definition:
In this case, you are selecting Thursday as the exception. If you want to use on, it might be better to rephrase your statement as "I am available on Mondays, Tuesdays, Wednesdays, and Fridays," which fits this definition:
The difference is slight and I doubt anyone but a pedant would fault you for using either one. As micsthepic mentioned, another option would be to omit the preposition entirely.