Definition from Wiktionary, the free dictionary
- Each of the four days customarily regarded as starting a new quarter year.
- in England: Lady Day (25 March), Midsummer Day (24 June), Michaelmas (29 September), Christmas Day (25 December).
- in Scotland: Candlemas (2 February), Whitsun (15 May), Lammas (1 August), Martinmas (11 November).
- Elsewhere, where the term "quarter day" is not used, January 1st, April 1st, July 1st and October 1st each mark the start of a quarter, especially a fiscal quarter.