A semimonthly event occurs twice a month. The term is commonly used in reference to the frequency of payrolls. For example, semimonthly payroll payments will result in 24 payments per year, typically on the 15th and last days of each month.
The concept differs from a biweekly event, which occurs once every two weeks. Thus, semimonthly events occur 24 times per year, and biweekly events occur 26 times per year.