Interest begins accruing the day after you deposit, and is paid out on a monthly basis (the first business day of each month). Interest compounds monthly, so after you receive your first interest payment, the next month interest will accrue on your original principal + interest earned!
If you deposit in the middle of the month, you will earn ~15 days worth of interest.
For example, if you deposited on the 15th you will earn ~14 days worth of interest. If you deposit on the 20th, you will earn ~9 days worth of interest. Each deposit is prorated based on the number of days that the specific deposit was in the account.