All six marathon majors scheduled this year will now be condensed into a six-week window, starting with the Berlin Marathon on Sept. 26 and ending with the New York City Marathon on Nov. 7 while the Olympics races will be held on Aug. 7-8. The field size for the world's most prestigious marathon, which generally draws over 30,000 runners from all over the world, will be forthcoming, the Boston Athletic Association (B.A.A.) said in a news release. "We announce the 2021 Boston Marathon date with a cautious optimism, understanding full well that we will continue to be guided by science and our continued collaborative work with local, city, state, and public health officials," said B.A.A. Chief Executive Tom Grilk.