Over 50 Massachusetts museums will open their doors for free on Saturday, Sept. 21, thanks to Smithsonian Magazine and its 15th annual Museum Day. For this one-day event, museums and cultural institutions all across the country will offer free admission to anyone with a Museum Day ticket. Here's what you need to do: visit smithsonianmag.com/museumday and search for the participating institution you'd like to visit. Then download your ticket, which grants free general admission for two people. Museums in all 50 states have signed up to participate in this year's event, and more than 1,300 museums are expected to join in...