Feliz Martes means “Happy Tuesday” in Spanish. It is used by many of twitter to represent their thoughts on Tuesday.
This help make posts from disparate posts and conversation around the same point easier to find and search. Using this hastag exposes the message of the tweet to everyone discussing the topic and looking at the messages.
Feliz Martes is popular on twitter and trending because it is used every Tuesday by users. Hastags that are used often are popular on twitter; I believe this explains why Feliz Martes is popular and trending on twitter.