A year holds 52 weekly pay periods, 26 biweekly, 24 semi-monthly, or 12 monthly, depending on the schedule you run. Two of those figures are fixed and two are not. Semi-monthly and monthly are anchored to calendar dates, so they deliver exactly 24 and 12 every year without exception. Weekly and biweekly are anchored to a day of the week, and because 52 weeks covers only 364 days, they drift. Every 11 to 12 years the drift produces an extra cycle.