What happens if I make one extra mortgage payment per year?

If you make an extra payment per year, your payment will remain the same but the number of payments left for the life of the loan will be reduced.
Can only tell you how many payments will be reduced if we know the current mortgage balance, the current principal and interest payment (or the balance when the loan started) and the interest rate. The higher the rate, the faster the payments will be reduced.
Also, need to know if your extra payment is made in a lump sum once year or if the extra payment is made one twelfth every month.

It is not advisible to go for yearly one tme payment at once also for biweekly payment is difficult to do. So monthly payment will moderate plan. Biweekly is very expensive plan also one time yearly payment is also some what tedious job. Adding 1/12th of a mortgage payment to the mortgage monthly payment is cheaper compared to tearly or biweekly payment.

A biweekly plan will work much the same way, but will most often cost a setup fee and produces basically the same effect

well said, john. additionally, it's important that anyone contemplating early payment on their mortgage know that prepayment is allowed in any amount - it does not have to work out to one monthly installment per year or any other such formula. a borrower can pay an extra $.01, $100, $500, $1000 or any other such amount. the only caveat to this is for those with prepayment penalties in their mortgages. if one exists, a borrower must acquaint himself with the stipulations to ensure that no penalty kicks in when making partial payments.