Elvis fans will gather next month for Branson's annual Elvis Festival. The three-day musical event will be held May 1 - 3.   Branson's Elvis Festival celebrates the memory and legacy of Elvis Presley with concerts, events, tribute shows and the Ultimate Elvis Tribute Artist Contest, one of only a few such competitions sanctioned by Elvis Presley Enterprises. The Ultimate Elvis Tribute Artist Contest brings in the best tribute artists from across the country, who are judged on