“Happy Birthday” is not one of their better-known tunes, but Bono and U2 will unveil an a capella version of the ditty today for Nelson Mandela. The band recorded the song at the close of a show at Philadelphia’s Lincoln Financial Field last week to be played at a Mandela Day celebration today hosted by Morgan Freeman at the Four Seasons restaurant. Guests expected at the shindig to highlight Mandela’s 67 years of public service — as well as his 93rd birthday — include Brian Williams, Billie Jean King, Charlie Rose and Star Jones.