Between June and September 2014, Lower Normandy hosted several large gatherings of international significance, each with specific and varied organizational contexts and audiences, which led local authorities to seek assistance from the Lower Normandy regional office of the Institute for Public Health Surveillance (InVS) (Cire) Normandy to assess the public health impact of these events and provide data to verify that healthcare and emergency response resources were adequately adapted. Routine public health surveillance was supplemented by dedicated public health monitoring systems involving several local health stakeholders. Throughout these gatherings, between 215 and 1,635 instances of people seeking medical care were recorded, and no specific health events were identified. An assessment of the effectiveness of the health surveillance systems and tools put in place highlighted the need to develop and adapt a health surveillance system to the specific context of each gathering, particularly with regard to the population involved, the environmental context, and the planned on-site healthcare and emergency response services. The responsiveness of partners and their active participation in these systems depend on their involvement in planning prior to the events and on thorough work to ensure the quality of the data transmitted on a daily basis. Finally, the epidemiological monitoring of potential public health events with a rapid onset of health impacts requires specific healthcare and emergency response arrangements to be defined in advance.