Labor Day closures Monday, Sept. 3 City offices, Mercer Island schools, the post office and Mercer Island Library will be closed. August 28, 2012 2:05 pm In honor of the Labor Day holiday on Monday, Sept. 3, there will be various closures around Mercer Island. City offices and Mercer Island schools will be closed. The post office and Mercer Island Library will also be closed.