Your account's billing day is the day that you initiated your plan — as monthly allowance credits reset within 24 hours of your billing day, it can be important for a creator to understand when this day falls on the calendar.
For monthly plan users, your billing day is the day that you receive your monthly charge. For annual users, this is the day of the month that you initiated your plan, but you will not receive any charge. As an example, a user that initiated their plan on the 20th of any given month would have a billing date of the 20th of each month.
If you don't remember when you initiated your plan, you can find your billing day by logging into the web app, clicking on your icon in the top right corner (if you don't have an icon set, this will display your initials), selecting Manage your plan from the menu that appears, and viewing the date associated with your last invoice:
Invoices are only provided for recurring subscription payments, which means there's no worry you'll get the wrong date by looking here. For this example user, their billing date would be the 9th of each calendar month. If they're an annual plan user, they'll get their credits within 24 hours of the 9th of each month without having to pay any renewal costs. If they're a monthly plan user, they'll get their credits within 24 hours of the the 9th of each month and pay for their subscription renewal on this date.