At Campaign level - spend limit

Your spend limit renewal will be set as “Monthly”
The spend limit will be set as your current monthly budget amount to replicate the current over-delivery protection of monthly budgets
Monthly spend limits renew on the 1st of every month

At Ad set level - budget strategy

Budget planning option

This option is available in Ad set Edition.

Your budget renewal date will become the 1st of every month. This differs from the current behavior of monthly budgets, which renews based on the start date of the budget.
Make sure to change your budget settings if this is not aligned with your spend strategy.
If your budget is scheduled to end before your ad set, your ad set end date will be adjusted to reflect the current budget end date. You can change the end date of your ad package if necessary.
For example, the current ad set has no end date, but the attached budget is scheduled to end on 6/30.
The ad set end date will be set to 6/30 to make sure that no additional delivery happens after 6/30.
This end date can be changed or removed at any time.

How to set budgets?