Interest is usually calculated as a daily accrual of the EOD balance, then applied monthly.
Some systems will actually do the accrual daily and store the balance in a shadow account, others will just calculate the interest when it is applied.
So if you had $100,000 in your account for just one day and nothing else for the rest of the year, at 5% interest you would earn 100000 * 0.05 / 365 = $13.69