You can begin your monthly subscription at any time. You can do so by entering the app menu, clicking on monthly subscriptions, pressing subscribe and confirming. Your billing cycle begins at the moment of subscription and your billing cycle runs 30 days from that day.
PLEASE NOTE: By confirming your subscription you will be IMMEDIATELY charged the first month's fee and your billing cycle begins. Only subscribe to the monthly service on the ay you want to use the service for the first time.