Congregationalists from all around Washington County will gather Sunday to mark a milestone. Their Association of Congregational Churches and Ministers was founded 175 years ago in Eastport. A service commemorating the event is planned for 3:30 p.m. Sunday, Nov. 14, at East Machias Congregational Meetinghouse. Congregational churches are fiercely independent, but the association of the two dozen such churches Down East has stressed common ground, particularly education. Special music and a dinner are planned for Sunday’s celebration.