Your invoice will be generated on the 1st day of every month for the usage of the previous month. If the direct debit option has been activated, the transaction will be done either on the 1st day of the month. The Billing cycle works on GMT time zone.
For example, the bill for the usage during the month of January from 1st to 31st will be generated on 1st of February. This will be an exception for the first month since the money will be prepaid for the first month to initiate the service.