The billing cycle starts the moment you sign up and make payment or upgrade to a higher level. You will then be billed each month on that date.
For example, if you make payment on September 15, then your payment applies to a full month of service going forward, all the way through to October 15.
If you know you won’t be sending during a certain billing period, you can feel free to pause your account during that time. Learn more about pausing here.