Microblog streams often contain a considerable amount of information about local, regional, national, and global events. Most existing microblog search capabilities are focused on recent happenings and do not provide the ability to search and explore past events. This paper proposes the problem of structured retrieval of historical event information over microblog archives. Rather than retrieving individual microblog messages in response to an event query, we propose retrieving a ranked list of historical event summaries by distilling high quality event representations using a novel temporal query expansion technique. The results of an exploratory study carried out over a large archive of Twitter messages demonstrates both the value of the microblog event retrieval task and the effectiveness of our proposed search methodologies.