The complete lineup for the Together Boston festival won’t be announced for a few days, but we do know that Mos Def will be among the performers in May. The “Mathematics” rapper, whose real name is Yasiin Bey, will take the stage at the Wilbur Theatre May 15. (The festival is scheduled for May 11-17.) “Yasiin Bey is certainly an idol of mine,” says David Day, Together Boston’s founder and creative director. “Not only do I have many of his songs memorized, but his activism and leadership within the African-American community is fully acknowledged. He is an icon for our times.” Together Boston, started in 2010, is a festival that honors music, art, and technology in Boston and Cambridge.