The choice might have had several levels of significance.Midsummer Day is also St. John's Day (June 24), the Feast of John the Baptist. Alice Hunt, in her recent study The Drama of Coronation: Medieval Ceremony in Early Modern England, suggests that there was an effort to identify the new king with Christ, as St. John prepares the way of the Messiah. It's also the longest day of the year, and traditionally celebrated with bonfires and feasting, and consequently fit well with Henry's desire to signal a real change from his father's regime. When I was researching this question, it was notable how the old king seemed to have celebrated a number of milestone events of his reign in the gloomier months of the year; coronation, October 30; marriage to Elizabeth of York, January; Elizabeth's coronation, November 25; Arthur's investiture as Prince of Wales, November; Catherine of Aragon's marriage to Arthur, in November. Not all events (Margaret Tudor got August for her proxy marriage to the Scots king), and clearly some of the schedule was under pressure from political and other considerations - Henry VII wanted to be crowned as soon as possible, and Catherine of Aragon's arrival was delayed by weather and other issues.Still, it's a sharp contrast with Edward IV, the "Sun in Splendour" who seems to have preferred summertime for royal family spectacles. He was crowned on what I would describe as the "St. Peter's Day long weekend," since the various sources I look at refer to St. Peter's Day as June 29 or 30, and the coronation taking place on either the Sunday (29th) or Monday (30th). This was very close to his grandson's choice of Midsummer Day, and I would bet on the Sunday being the correct date, because Henry VIII was crowned on a Sunday that was also Midsummer Day. Sundays seem to be the preferred weekday for coronations, for both kings and queens consort. Possibly Henry VIII wanted to align himself with his Yorkist grandfather rather than his Lancastrian father at his coronation, and chose the date accordingly. He was "lucky" that his father died in April, but there were other dates he could have chosen in between. Hunt notes that monarchs often selected religious holidays for important events like weddings and coronations. Anne Boleyn was first presented as Queen at Easter, for example, and crowned on Whitsunday.Finally, Midsummer Day has a lot of fertility ritual associated with it. Possibly the double coronation of king and queen on this date was a conscious effort to invoke that symbolism for the watching audience. Henry might also have been signalling that his queen would have a more prominent role than his mother (perhaps Henry VII scheduled his wedding and wife's coronation for the winter months to discourage Yorkist demonstrations).
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