According to David Wygant of DatingAdvice.com, "MCM" stands for "man crush Monday." As of 2013, the social media trend involves users posting about their current male crush on Mondays.
According to the International Business Times, a similar trend, "WCW," or "woman crush Wednesday," takes place on social media on Wednesdays. Users of Facebook, Twitter, Instagram and Tumblr participate in the trend by posting photos of a woman they admire or find attractive. According to the source, both men and women participate in this "mini social media holiday," and the crushes posted may often be a celebrity or a love interest.