I depends what you want to say. At the moment, it is unclear. Do you mean that you are not free on Thursdays? If so, 'besides' is wrong. 'Besides' means 'in addition to'. If you have time on Mondays, Tuesdays etc but NOT Thursdays, you need to say: I have time after 8 pm every day apart from Thursday. or I have time after 8 pm every day except for Thursday.

Correct