Over at Celestia’s place, she commented that she had the day off and not even her Japanese co-workers knew precisely what the holiday was. This led to the discovery of two rather intriguing clauses in the law governing national holidays:

国民の休日

Kokumin no Kyuujitsu, "national holiday", defined as a weekday between two holidays. (this year, the day between Respect The Elderly Day and the Fall Equinox)

振替休日

Furikae Kyuujitsu, "transfer holiday", defined as the first non-holiday weekday following a holiday that fell on a Sunday. (this year, Constitution Memorial Day was a Sunday, but the next two days were already holidays (Green Day and Children's Day), so Wednesday became a holiday)