Semi-monthly Payroll implies that salaries are given out twice monthly, and the employees receive their salaries on daily recurring dates, such as the 15 th and the final day of the month. The outcome of this is that there are 24 payments per annum.
It is not like biweekly payroll that is paid twice a week (26 times in one year). The semi-monthly pay periods might be unfixed, in terms of days.