Flags often are raised as a form of government speech on government-owned property.  The government has the right to control its own speech including what flags it flies.  However, in this case, Boston allowed private groups to fly their flags from government-owned poles about 20 times a year.  Those flag-flying days were not government speech.  So on those days, the First Amendment barred the government from refusing to allow religious groups to fly flags expressing their religious convictions.)