SEASON KICKOFF: While today is a solemn holiday for many, it also means big business for flag sellers. Most businesses say that Memorial Day "kicks off" their busy season, which will run from now through the Fourth of July. . . . "In the spring and summer people spruce up and replace old flags," says Corki Whitford of Nikki's Flags in Newport Beach. Most local sellers say they double their stock in both flags and poles between now and the Fourth.