If an eligible person who is hospitalized pursuant to the Act, or his legal guardian, spouse, or adult next of kin, requests his release, such request shall be granted by the Administrator if his best interests will be served thereby, or by the head of the hospital if he is found not to be in need of hospitalization by reason of mental illness. The right of the administrator or the head of the hospital, to refuse such request and to detain him for care and treatment shall be determined in accordance with laws governing the detention, for care and treatment, of persons alleged to be mentally ill as may be in force and applicable generally in the State in which such hospital is located, but in no event shall the patient be detained more than forty-eight hours (excluding any period of time falling on a Sunday or a legal holiday observed by the courts of the State in which such hospital is located) after the receipt of such request unless within such time (a) judicial proceedings for such hospitalization are commenced or (b) a judicial extension of such time is obtained, for a period of not more than five days, for the commencement of such proceedings.