Collar day

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a day of great ceremony at the English court, when persons, who are dignitaries of honorary orders, wear the collars of those orders.
- Johnson.

See also: Collar

Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
References in periodicals archive ?
Too sensitive to frolic with bigger boys on "collar day," Forest endures teasing as a "hare heart" after he vomits during the hanging of a notorious highwayman.
Fantastically, yesterday was a Collar Day. It said so in the brochure, sorry programme, to accompany the State Opening of Parliament.
But here - to mark what would have been Lennon''s 70th birthday, are no fewer than 32 images - only one without the tell-tale round spectacles, and dating from the Fab Four cutaway collar days (when JL, out of camera shot, actually wore thick black horn-rimmed glasses).