Every employer covered by this Act shall post and keep posted, in conspicuous places on the premises of the employer where notices to employees are customarily posted, a notice, to be prepared or approved by the Director of Labor, summarizing the requirements of this Act and information pertaining to the filing of a charge. The Director shall furnish copies of summaries and rules to employers upon request without charge. Any employer that fails to post the required notice may not rely on the provisions in subsection (b) of Section 20 to claim that the employee failed to inform the employer that she or he wanted or was eligible for leave under this Act.