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 <a class="localLink" href="http://schema.org/OpeningHoursSpecification">OpeningHoursSpecification</a>. 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.