Is it okay to go to an 18th the weekend before exams?

Everyone needs to be motivated to study properly. If you work hard but have something to look forward to as well then you are more likely to learn better. So I think as long as you keep to what you say and work really hard in the week and on Saturday then it would be good for you to then go out and have some fun! Alternatively if you are still doubting it then you could always arrange to meet up after your exams somewhere with these friends?