Definition:Semi-monthly describes a situation where an event occurs twice per month. This usually means that workers are paid on the first and fifteenth or alternatively on the fifteenth and thirtieth, of each month. The end result is 24 pay periods for each year. The difference between semi-monthly and biweekly pay is that the latter happens every two weeks and there are 26 installments each year. Workers are able to plan their budget with regular semi-monthly pay, but it is more difficult for hourly workers because their work schedule might not be the same every time.