Linguix uses a feature called recurring payments, which means your subscription automatically renews after the end of the billing period. Plan renewal will be made at the rates states on the subscription page at the moment of the signup.
Your account will be charged monthly, quarterly, or annually depending on your chosen option. Billing date is the date you signed up. If this date does not exist in some months (like 29th of February), you will be billed on the last day of that month.