Voters will go to the polls July 19 to decide whether a leaf-blower ban approved by Town Meeting last month should be overturned. The special election will be held from 2 to 8 p.m., and will require participation by at least 20 percent of Arlington’s registered voters in order to overturn the new bylaw. Town Meeting members voted to ban gas-powered leaf blowers between May 15 and Oct. 15 each year, based on concerns about noise and potentially harmful debris kicked up by the power tools. A group of landscapers and residents gathered enough signatures on a petition to call the special election, and requested that it be held in September. But the town counsel, Juliana Rice, said Monday that under state law the special election must be held “forthwith” after a petition is submitted, and the town cannot put the vote off until the fall. The election is expected to cost the town about $30,000. The last day to register to vote is June 28.