On April 16, 1964, the day before Shea Stadium officially opened, Bill Shea christened the Mets' new home with two symbolic bottles of water: one from the Gowanus Canal, near Ebbets Field, the former home of the Brooklyn Dodgers and one from the Harlem River, near the Polo Grounds, where the New York Giants had played and later the Mets during their first two years. The next morning, April 17th, construction workers were painting outfield signs and fresh sod was being laid in the outfield as the teams took batting practice. (The Mets lost, 4-3, to Pittsburgh that afternoon.)
I hardly need to say that the Mets were originally conceived as a balm to the wounds felt by Giant and Dodgers fans whose National League teams were stolen from them (moving to California) in the late 50s, in moves that have often and can really only be interpreted as white flight.
By the way, my friend Peter Tarr, who passed along this factoid to me, himself attended that first Shea game, April 18, 1964, the first of many, many losses Pete has endured.