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

byDay

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

Thing > Property > byDay

Defines the day(s) of the week on which a recurring Event takes place
Values expected to be one of these types
DayOfWeek
Used on these types
Schedule

Source

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

Schema Version 3.3