Event takes place. May be specified using either <a class="localLink" href=https://schema.org/"/DayOfWeek">DayOfWeek, or alternatively <a class="localLink" href=https://schema.org/"/Text">Text conforming to iCal's syntax for byDay recurrence rules." />
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byDay

A Schema.org Property

This term is in the "new" area - implementation feedback and adoption from applications and websites can help improve our definitions.
Thing > Property :: byDay
Defines the day(s) of the week on which a recurring Event takes place. May be specified using either DayOfWeek, or alternatively Text conforming to iCal's syntax for byDay recurrence rules.

Values expected to be one of these types

Used on these types

Source

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