Fourth of July celebrations hailing America’s 250th birthday will be held across Riverside County Saturday evening, preceded by parades and other festivities. Corona’s traditional “Main Street U.S.A Parade” will begin at 9 a.m. Saturday at Ontario and Main streets, featuring law enforcement units, fire engines, marching bands, military hardware and show animals. The procession, which typically draws around 4,000 spectators, will travel about a m…