Generally, interest on a mortgage is calculated daily and then charged at the end of each month. To make these calculations, your mortgage lender will take the outstanding loan amount at the end of each business day and multiply it by the interest rate that applies to your loan.
This amount is then divided by the number of days in a year - 365 (or 366 in a leap year). At the end of the month, your lender will add together the daily interest charges for each day. This is the monthly interest amount identified on your mortgage statement. If you make weekly or fortnightly mortgage repayments, then this amount is charged accordingly.
Alongside interest rate changes, the interest amount you are charged can also change depending on the number of days in the month. As interest is generally calculated every day, the more days in the month the higher the interest amount.