"it's an Arches day"
St. Mary le Bow
GNU Free Documentation LicenseSt. Mary le Bow - Credit: Wikimedia Commons

An “Arches day” was a day when the Court of Arches, the ecclesiastical court of appeal for the province of Canterbury, was in session.

It came to be called the Court of Arches in reference to the arches of St. Mary le Bow Church.