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byMonth

This term is proposed for full integration into Schema.org, pending implementation feedback and adoption from applications and websites.

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