Your bill will commence from the 1st day of the month until the final day of the month. Every monthly invoice is generated on the 5th day of each month with the due date at the 15th of every month (Friday before if the 15th falls on a weekend).
A pro-rata access fee for the first month based on the number of days you are connected is charged on your first bill, however, you will get get the full inclusions for the month, including all call and data allowances. For example, if you join on the 15th day of the month you will pay a pro-rata rate for the amount of days remaining in that calendar month and a month in advance but will still get all the call and data inclusions for that part month.