29 C.F.R. § 778.102

Since there is no absolute limitation in the Act (apart from the child labor provisions and regulations thereunder) on the number of hours that an employee may work in any workweek, he may work as many hours a week as he and his employer see fit, so long as the required overtime compensation is paid him for hours worked in excess of the maximum workweek prescribed by section 7(a). The Act does not generally require, however, that an employee be paid overtime compensation for hours in excess of eight per day, or for work on Saturdays, Sundays, holidays or regular days of rest. If no more than the maximum number of hours prescribed in the Act are actually worked in the workweek, overtime compensation pursuant to section 7(a) need not be paid. Nothing in the Act, however, will relieve an employer of any obligation he may have assumed by contract or of any obligation imposed by other Federal or State law to limit overtime hours of work or to pay premium rates for work in excess of a daily standard or for work on Saturdays, Sundays, holidays, or other periods outside of or in excess of the normal or regular workweek or workday. (The effect of making such payments is discussed in §§ 778.201 through 778.207 and 778.219.)