Besides the day on which you subscribe, your credit card will be automatically charged every month for monthly plans. As for yearly plans, you are charged one day prior to the subscription date the following year. For instance, if you subscribe on 25-08-2011, your card is charged on 24-08-2012 for your next year's subscription. This is so much easier because you wouldn't need to worry over subscription renewals et al.
Since this process is automated, you will be charged automatically until you change your subscription to 'Free'.
Zoho Invoice FAQ