Identifying a clear relationship between rebel group structures and the use of violence faces the challenge that group structures rarely change over time. We exploit the analytical advantage provided by religious holidays to address this issue using the principal-agent framework. Religious holidays serve as a focal point and reduce group coordination costs, but also raise the societal costs of violence. We argue the principal of rebel groups is more sensitive to the increased societal costs than the agents and thereby attempts to restrain the agents from attacking during religious holidays. However, the extent to which these attempts are successful depends on the group’s institutionalization level. We test the theory by first conducting microlevel analysis of Islamic separatist groups in three Southeast Asian countries and then analyzing a cross-sectional sample of Islamic rebel groups. Results show that highly institutionalized groups that have a central command system and control over constituent groups are less likely to attack during long religious holidays than on other days, and vice versa for weakly institutionalized groups.