The day of the week, e.g. used to specify to which day the opening hours of an OpeningHoursSpecification refer.
Originally, URLs from [GoodRelations](http://purl.org/goodrelations/v1) were used (for [Monday](https://schema.org/Monday), [Tuesday](https://schema.org/Tuesday), [Wednesday](https://schema.org/Wednesday), [Thursday](https://schema.org/Thursday), [Friday](https://schema.org/Friday), [Saturday](https://schema.org/Saturday), [Sunday](https://schema.org/Sunday) plus a special entry for [PublicHolidays](https://schema.org/PublicHolidays)); these have now been integrated directly into schema.org.
