26 CFR 31.3401(b)-1 - Payroll period.
(a) The term payroll period means the period of service for which a payment of wages is ordinarily made to an employee by his employer. It is immaterial that the wages are not always paid at regular intervals. For example, if an employer ordinarily pays a particular employee for each calendar week at the end of the week, but if for some reason the employee in a given week receives a payment in the middle of the week for the portion of the week already elapsed and receives the remainder at the end of the week, the payroll period is still the calendar week; or if, instead, that employee is sent on a 3-week trip by his employer and receives at the end of the trip a single wage payment for three weeks' services, the payroll period is still the calendar week, and the wage payment shall be treated as though it were three separate weekly wage payments.
(b) For the purpose of section 3402, an employee can have but one payroll period with respect to wages paid by any one employer. Thus, if an employee is paid a regular wage for a weekly payroll period and in addition thereto is paid supplemental wages (for example, bonuses) determined with respect to a different period, the payroll period is the weekly payroll period. For computation of tax on supplemental wage payments, see § 31.3402(g)-1.
(c) The term payroll period also means the period of accrual of supplemental unemployment compensation benefits for which a payment of such benefits is ordinarily made. Thus if benefits are ordinarily accrued and paid on a monthly basis, the payroll period is deemed to be monthly.