A bi-monthly pay period is a payroll schedule where employees receive payment twice per month, usually on the same calendar days. It is also referred to as a semi-monthly pay period. When an employer uses a bi-monthly pay period, it issues 24 paychecks per employee in a year. While bi-monthly technically means once every two months, in the payroll world, it has come to mean twice per month, or semi-monthly. Pay dates are typically the first and 15th of the month, or the 15th and the last day of the month, for a total of 24 pay runs per year.
Source - https://accountingbyte.com/semi-monthly-vs-bi-weekly/