Do you use Lucky feat before or after an attacker has rolled?

@JeremyECrawford #DnD in 17hours from now, and I need to know before getting Lucky feat.Do you use it before or after an attacker has rolled

— Peter Korntved (@PKorntved) September 27, 2016

Lucky lets you choose between using your roll or your attacker's. The attacker must make a roll for you to make this choice. #DnD https://t.co/CMS3UFxM4U

— Jeremy Crawford (@JeremyECrawford) September 27, 2016