FRIDAY: >> The city’s free concert series follows MC Hammer with a rare appearance by salsa legend Willie Colon, 7-9 p.m. at Woodrow Wilson Center. >> President Nixon’s White House counsel John Dean will be at Politics and Prose to discuss his book, Broken Government, which examines “the institutional damage he believes the Republican Party has inflicted on the executive, legislative, and judicial branches of government during the Bush administration.” 7 p.m. He’ll also be…