When am I billed for my subscription?

Memberships are billed on a monthly basis. For monthly subscriptions, billing date can be a particular date, from 1st to 31st of the month. Your billing date is based on the sign up date, also known as activation, of the subscription. 

What would be the monthly billing date of customers whose billing date fall on the 31st of the month?

For ‘End of the Month' cases, if a selected date is not present in a month (say, the 31st) for billing, then the preceding date is selected. The renewal shifts back to the original date in the subsequent month, if available.

The customer has signed up for a subscription that renews on the 31st of every month.
The renewal of the subscription will fall on Jan 31, February 28, March 31 and so on, i.e., the subscription will renew on the last day of the month.