I believe that you have accidently left out "to" after "attend." I have something to attend TO tonight. = I have to do something tonight. / I have some business that I have to take care of tonight. Compare: I have a party to attend tonight. = I plan to go to a party tonight.

Now I can guess the meaning for I couldn't help reading it over and over again ever since I came across it. Maybe it means 'I have something to accompany tonight'

What is the meaning of this sentence 'I have something to attend tonight?'

what is matter?