My habit is to cancel right away when I start a subscription. You can still use the service for the month you paid for, and it's easy to restart it if you do still find yourself attempting to use it a month later.
- Payment cycle closes always on the 1st of the month (which means, if you sign up on the last day of the month, you get 1 day of service for full month of payment. No proration, or anything.)
- If you cancel the service, you'll immediately lose access, doesn't matter if it's 1st day of the month, or very end of the month, it's gone. To reenroll, you often have to pay again.