ISO 20121:2012 Event Sustainability Management System is closely connected to ISO 26000 and therefore highlighted at this ISO 26000 website.
The standard contains requirements for an event sustainability management system. It is possible to voluntarily demonstrate conformity by using first party (self-determination and self-declaration), second party (confirmation of conformance by parties having an interest in the organization, such as clients, or by other persons on their behalf), or an independent third party (e.g. a certification body).
More information is available at http://www.iso20121.org/ .