Somerville will host a public meeting Thursday to discuss the impending sale of the historic Somerville Main Post Office in Union Square. The meeting will be convened by the Somerville Historic Preservation Commission at 7 p.m. in the third-floor conference room of City Hall, 93 Highland Ave. The US Postal Service is selling the Post Office at 237 Washington St. and transferring distribution operations to Chelsea while opening a smaller retail office on Bow Street in Union Square. Built in 1900, the Washington Street Post Office was placed on the National Register of Historic Places in 1986 and designated as a local historic district in July 2013. Because of the building’s historic designations, any alterations to its exterior would be subject to review by the Historic Preservation Commission. The Postal Service has also agreed to restore a 1939 mural in the building’s lobby whether it remains in its current location or is moved to another public space, according to the city.