MC2 CLEF 2017 lab deals with how cultural context of a microblog affects its social impact at large. This involves microblog search, classification, filtering, language recognition, localization, entity extraction, linking open data, and summarization. Regular Lab participants have access to the private massive multilingual microblog stream of The Festival Galleries project. Festivals have a large presence on social media. The resulting mircroblog stream and related URLs is appropriate to experiment advanced social media search and mining methods. A collection of 70,000,000 microblogs over 18 months dealing with cultural events in all languages has been released to test multilingual content analysis and microblog search. For content analysis topics were in any language and results were expected in four languages: English, Spanish, French, and Portuguese. For microblog search topics were in four languages: Arabic, English, French and Spanish, and results were expected in any language.