It depends, but many services tend to bill on a single day each month and pro-rate new sign-ups based on the remaining days.

There are several advantages to paying on the say day every month, but mostly it allows the business to deal with recurring billing issues in one swoop. If 1% of clients will have problems (credit card declined, expired, "what is this Blah,Inc" who charged me, etc), then dealing with all of those at once will be easier for staff.

It also help with cashflow management, since you will get a predicitable lump sum of money on the same day each month.

Most services charge me on the same monthly day on which I made my first payment. If its something like pay per use, sometimes they charge bimonthly or so. To my experience both customers and providers prefer this schedule.

Providers -

• Distributes work load throughout the month so that your staff is not overburdened at some days of the month and has no work on other days of the month.
• Adjustment of amounts to a specific date is difficult for new customers. They get confused about why they are being charged arbitrary amounts at arbitrary dates. People are weak at math.

Customers -

• Many people pay on behalf of their bosses, wives, kids and so forth. So all they see is some amount and a date on their cc billing.
• Find it easy to know that they will be charged so and so, around the same day every month. So they can make decisions/plans accordingly.
• Even if they "upgrade" or "downgrade" services, they expect being charged on the same day. Like if they upgrade you charge extra amount immediately.

Always stick to one schedule and clearly let your customers know that schedule.