#OnThisDay in 2009, "We Are One" concert at the Lincoln Memorial in Washington prior to #BarackObama's inauguration attracts an estimated crowd of 400,000; performers include: #Beyoncé, #JonBonJovi, #GarthBrooks, #MariahCarey, #JoshGroban, #HerbieHancock, #JohnLegend, #PeteSeeger and #BruceSpringsteen, #U2, and #StevieWonder.