Southampton Town is celebrating its 375th birthday with a yearlong series of lectures, exhibitions, walking tours, parties, and dinners at locations throughout the town, beginning today at 3 p.m. with a screening of “The Manors of Long Island” at the Rogers Memorial Library. Next Thursday at 3 p.m., also at the library, Georgette Grier-Key, director and curator of the Eastville Community Historical Society, will discuss the influence of African-Americans locally and in the larger society.