Depends where you live. In the US, you have both Federal and state laws to look at.
Federal labor law on overtime:
The federal overtime provisions are contained in the Fair Labor Standards Act (FLSA). Unless exempt, employees covered by the Act must receive overtime pay for hours worked over 40 in a workweek at a rate not less than time and one-half their regular rates of pay. There is no limit in the Act on the number of hours employees aged 16 and older may work in any workweek. The Act does not require overtime pay for work on Saturdays, Sundays, holidays, or regular days of rest, unless overtime is worked on such days.
The Act applies on a workweek basis. An employee's workweek is a fixed and regularly recurring period of 168 hours — seven consecutive 24-hour periods. It need not coincide with the calendar week, but may begin on any day and at any hour of the day. Different workweeks may be established for different employees or groups of employees. Averaging of hours over two or more weeks is not permitted. Normally, overtime pay earned in a particular workweek must be paid on the regular pay day for the pay period in which the wages were earned.
State laws can be more restrictive on when OT must be paid.