This stands for any day that is a public holiday; it is a placeholder for all official public holidays in some particular location. While not technically a "day of the week", it can be used with OpeningHoursSpecification. In the context of an opening hours specification it can be used to indicate opening hours on public holidays, overriding general opening hours for the day of the week on which a public holiday occurs.
This class is derived from the GoodRelations Vocabulary for E-Commerce, created by Martin Hepp. GoodRelations is a data model for sharing e-commerce data on the Web that can be expressed in a variety of syntaxes, including RDFa and HTML5 Microdata. More information about GoodRelations can be found at http://purl.org/goodrelations/.