Researchers have argued that Twitter has potential to support high-quality professional development (PD) that can respond to teachers’ questions and concerns just in time and “on the spot.” Yet, very little attention has been paid to instances where Twitter has made just-in-time learning possible. In this paper, we examine one instance of timely professional development on Twitter, in which 3,598 users used an educational hashtag—#educattentats—to create a temporary affinity space supporting French teachers preparing to discuss recent terrorist attacks with their students. We describe this just-in-time PD by focusing on participants and modes of participation, the spread of the hashtag in its first hours and the growth and eventual decline of the hashtag over the course of 28 days. The results of this study suggest that #educattentats served effectively as just-in-time professional development for teachers. Implications for research and practice are discussed.