27 CFR 70.244: Payroll period.

For purpose of determining the amount of wages, salary or other income exempt from levy under 26 U.S.C. 6334(a)(9):

(a) Regularly used calendar periods. In the case of wages, salary or other income paid to the taxpayer on the basis of an established calendar period regularly used by the employer or other person levied upon for payroll or payment purpose (e.g., daily, weekly, biweekly, semimonthly, or monthly), that period is the taxpayer's payroll period.

(b) Amounts paid on recurrent but irregular basis. In the case of wages, salary, or other income paid to the taxpayer on a recurrent but irregular basis, the first day of the taxpayer's payroll period is that day following the day upon which the wages, salary, or other income were last paid to the taxpayer. The last day of the payroll period is that day upon which the current payment becomes payable to him or her. However, in any case in which:

(1) Amounts are paid to the taxpayer on a recurrent but irregular basis, and

(2) The last payment was paid to the taxpayer more than 60 days before the current payment becomes payable, the current payment will be deemed a one-time payment (see paragraph (c) of this section).

(c) Nonrecurrent payments. In the case of wages, salary or other income paid to the taxpayer on a one-time basis, the taxpayer's payroll period is deemed to be weekly (i.e., the 1-week period ending on the day of payment).