What is an apéro actually? it is an event where people gather to chat, drink and dance outside! It is quite cool as the concept is quite recent and yet has so many fans! It takes only place in Summer and the places are to be disclosed a few days before the event itself. Be sure to check their website to know when it takes place!
If you want to enjoy the good weather (that is quite rare in Brussels) whilst listening to music and seeing a lot of young people in Brussels, it's definitely the right choice!
It usually is within Brussels and accessible with public transportations. Make sure to check the website to know what is going on, where and when.