Section 21C1/2. An employee of a city or town which accepts the provisions of this section who is a veteran as defined in clause Forty-third of section seven of chapter four or is a member of a department of war veterans listed in section seventeen of chapter eight, and who desires to participate in a Veterans Day or Memorial Day exercise, parade or service, may be allowed and granted a reasonable and sufficient period of time off from his duties to attend and participate in such services or exercises. This section shall not apply to employees whose duties and services are critical and essential to the public safety as determined by ordinance or by-law, and the city or town may determine whether such time shall be with or without pay.