Background: Mass gatherings cause a need for multi-authority preparedness in order to ensure the safety of the event participants and to minimize delays in response for emergencies. Rescue authorities are key players in the pre-planning phase; however, their own point of view regarding all aspects of preparedness for mass gatherings is not well known. The aim of this study was to investigate what factors, according to the rescue authorities, need to be considered when preparing for mass gatherings. Method: Semi-structured thematic interviews were carried out with the rescue authorities involved in the mass gathering planning process (n = 15). The transcribed material was analyzed using inductive content analysis. Results: Three main categories emerged from the interviews: (1) co-operation in the pre-planning phase, (2) factors to be noted in the emergency plan, and (3) actions during the event. These categories were divided into 11 generic categories, which were further divided into 42 sub-categories. Conclusion: Rescue authorities recognized various factors considering preparedness for mass gatherings. Knowledge considering the dispersion of operative workload during the event needs further investigation in order to facilitate the effective use of limited operative resources.