(a) Each municipality shall include a requirement in any collective bargaining agreement executed on or after April 26, 2000, that all nonessential municipal offices shall be closed on any day designated as Martin Luther King Day pursuant to section 1-4 .
(b) Any municipality that did not observe the Martin Luther King Day legal holiday on January 17, 2000, by closing all nonessential municipal offices shall close all such nonessential municipal offices on any day designated as Martin Luther King Day pursuant to section 1-4 .