The Lestrange family was hosting a party in honour of Rodolphus 17th birthday. It was supposed to be a fun party, something he would enjoy. But he knew better. He knew perfectly that his father only did this so he could have a chance to choose a wife before he was forced with someone he didn't even know. He also knew that his younger brother was in the same position. He didn't have anything to do with this, but it's clear that their father also wanted him to get in action too.