29 C.F.R. § 825.304 : Labor — REGULATIONS RELATING TO LABOR — WAGE AND HOUR DIVISION, DEPARTMENT OF LABOR — OTHER LAWS — THE FAMILY AND MEDICAL LEAVE ACT OF 1993 — EMPLOYEE AND EMPLOYER RIGHTS AND OBLIGATIONS UNDER THE ACT — Employee failure to provide notice

(a) Proper notice required. In all cases, in order for the onset of an employee's FMLA leave to be delayed due to lack of required notice, it must be clear that the employee had actual notice of the FMLA notice requirements. This condition would be satisfied by the employer's proper posting of the required notice at the worksite where the employee is employed and the employer's provision of the required notice in either an employee handbook or employee distribution, as required by §825.300.
(b) Foreseeable leave-30 days. When the need for FMLA leave is foreseeable at least 30 days in advance and an employee fails to give timely advance notice with no reasonable excuse, the employer may delay FMLA coverage until 30 days after the date the employee provides notice. The need for leave and the approximate date leave would be taken must have been clearly foreseeable to the employee 30 days in advance of the leave. For example, knowledge that an employee would receive a telephone call about the availability of a child for adoption at some unknown point in the future would not be sufficient to establish the leave was clearly foreseeable 30 days in advance.
(c) Foreseeable leave-less than 30 days. When the need for FMLA leave is foreseeable fewer than 30 days in advance and an employee fails to give notice as soon as practicable under the particular facts and circumstances, the extent to which an employer may delay FMLA coverage for leave depends on the facts of the particular case. For example, if an employee reasonably should have given the employer two weeks notice but instead only provided one week notice, then the employer may delay FMLA-protected leave for one week (thus, if the employer elects to delay FMLA coverage and the employee nonetheless takes leave one week after providing the notice ( i.e., a week before the two week notice period has been met) the leave will not be FMLA-protected).
(d) Unforeseeable leave. When the need for FMLA leave is unforeseeable and an employee fails to give notice in accordance with §825.303, the extent to which an employer may delay FMLA coverage for leave depends on the facts of the particular case. For example, if it would have been practicable for an employee to have given the employer notice of the need for leave very soon after the need arises consistent with the employer's policy, but instead the employee provided notice two days after the leave began, then the employer may delay FMLA coverage of the leave by two days.
(e) Waiver of notice. An employer may waive employees' FMLA notice obligations or the employer's own internal rules on leave notice requirements. If an employer does not waive the employee's obligations under its internal leave rules, the employer may take appropriate action under its internal rules and procedures for failure to follow its usual and customary notification rules, absent unusual circumstances, as long as the actions are taken in a manner that does not discriminate against employees taking FMLA leave and the rules are not inconsistent with §825.303(a).

29 C.F.R. § 825.304