If you've used a promotional code when purchasing a subscription, you may have been provided a month or more for free. However when the subscription is created, the system still requires the billing periods to be established even if you are not charged. To do this, the payment system used by OnSong will provide you a month of access. Once the next month is reached, the payment is fulfilled by the promotional code and the next billing period is entered. At each of these "payments", the expiration date of the account is incremented.
Once the promotional period expires, the payment method on file will be charged and the subscription will continue. If you've signed up to pay annually, your expiration date should then advance forward one year. If you opt to pay month-to-month, the expiration date will advance forward one month.
While OnSong does provide a grace period, it's recommended that you purchase OnSong annually if you are expecting to be disconnected from the Internet for a prolonged period of time to ensure your subscription remains active.