Terry F on 29 Sep 2005  •  Link

Collar Days

Knights Grand Cross (in the generic sense) wear collars of orders of knighthood on "collar days".... In addition, collars are worn when Her Majesty opens or prorogues Parliament, and by those taking part in the ceremony of the introduction of a peer in the House of Lords. They are not worn after sunset, nor when mounted on ceremonial parades such as Trooping the Colour unless instructed otherwise.... http://www.cam.ac.uk/societies/cu…

John York on 3 Feb 2016  •  Link

"Collar days are designated days on which the collar forming part of the insignia of certain members of orders of knighthood may be worn. Collars are special large and elaborate metal chains worn over the shoulders, hanging equally in front and back, often tied with a bow at the shoulders, with a distinctive pendant attached to the front."
https://en.wikipedia.org/wiki/Col…
From the list quoted in the 1831 London Gazette there are 32 days listed (excluding royal birthdays) and these are nearly all religious festivals where members of the court would have been expected to attend chapel.
https://www.thegazette.co.uk/Lond…...
Whilst in the 17th century this would principally relate to the British Orders of Knighthood no doubt foreign ambassadors to the court would wear the orders of their respective countries.