This article will show you how to enable Retroactive Start Dates. This allows for Fixed Payment Dates.
If you enable this Organization Option, that can solve a troubleshooting issue where billing on the first of the month within a 12 month/year-long membership skips the second month of billing. This happens because the first month might technically start on the next month or on the current month, depending on how it is configured. If you enable Retroactive Start Dates then add a Pro-Rated Discount to the membership, it will start the current month and pro-rate for the amount of time they missed. Then, your next payment would be the next first.
Instructions
Navigate to Configuration > Organization > Organization Options
Search for Allow retroactive start dates
From the Value dropdown, choose Enabled
Click Save Settings
