Manhattan's Fifth Avenue will become a river of green and thunder with the sounds of more than 100 marching bands Saturday in the 257th edition of New York City's St. Patrick's Day parade.The parade starts at 11 a.m. and typically lasts for nearly six hours, taking an estimated 150,000 marchers on a 1.4-mile route past Central Park, St. Patrick's Cathedral and Trump Tower.A big event in the city since the mid-1800s, the parade has been a celebration of Irish culture and Irish immigrants, who once faced nativist calls for their exclusion from the workforce — and from the country — when they began arriving in the city in huge numbers during the Irish Potato Famine.For the 167th time, the lead group marching in the parade will be 800 members of the 1st Battalion, 69th Infantry Regiment, of the New York Army National Guard. The regiment, once predominantly made up of Irish immigrants, first led the parade in 1851 as a deterrent to anti-immigrant violence.This year's grand marshal is Loretta Brennan Glucksman, chairman of The American Ireland Fund, a group that has raised millions of dollars for philanthropic projects in Ireland, including funding for integrated schools for Catholic and Protestant children in Northern Ireland.She'll be riding along the parade route in a Central Park horse carriage driven by a family friend.Democratic Mayor Bill de Blasio, who took office vowing to ban horse carriages from the park to promote the humane treatment of animals, is also marching near the front of the parade with the city's police commissioner, James O'Neill.