Religious festivals attract a large number of pilgrims from worldwide and are a potential risk for the transmission of infectious diseases between pilgrims, and to the indigenous population. The gathering of a large number of pilgrims could compromise the health system of the host country. The threat to global health security posed by infectious diseases with epidemic potential shows the importance of advanced planning of public health surveillance and response at these religious events. Saudi Arabia has extensive experience of providing health care at mass gatherings acquired through decades of managing millions of pilgrims at the Hajj. In this report, we describe the extensive public health planning, surveillance systems used to monitor public health risks, and health services provided and accessed during Hajj 2012 and Hajj 2013 that together attracted more than 5 million pilgrims from 184 countries. We also describe the recent establishment of the Global Center for Mass Gathering Medicine, a Saudi Government partnership with the WHO Collaborating Centre for Mass Gatherings Medicine, Gulf Co-operation Council states, UK universities, and public health institutions globally.