Savings accounts accrue interest on a monthly basis. They may be deposited into and withdrawn from at any time.
Interest rate is represented by APY, the Annual Percentage Yield. The monthly interest rate is represented by: (1 + APY)(1/12) - 1
The monthly rate is calculated based on the Average Daily Balance (ADB) of your account. This is determined by: (sum of account at end of every day of month) / (number of days in month)
The interest payment is calculated as (Interest Rate) * (Average Daily Balance). Payments are based on the ADB, not on the balance at the time interest is run.