core + pending (Pending Extension): these terms are pending wider review. Feedback is welcomed!

byMonth

Defined in the pending.schema.org extension.
Canonical URL: http://schema.org/byMonth

Thing > Property > byMonth

Defines the month(s) of the year on which a recurring Event takes place. Specified as an Integer between 1-12. January is 1.
Values expected to be one of these types
Integer
Used on these types
Schedule

Source

https://github.com/schemaorg/schemaorg/issues/1457

Schema Version 3.3