For those newer to the movement, like Sam Jandl of Boston, who stepped up to help organize Sunday’s event on Boston Common, the connection to others around the country has provided a morale boost as well as practical advice, like which trash bags to buy to protect posters from rain. “For me, it helps to understand how large a community of people you’re in, people who understand the issue and are willing to put a coat on and commit to coming out,” Ms. Jandl said. The first Women’s March, which was held in January 2017, the day after Mr. Trump’s...