I suppose that, if ours were an acknowledged monarchy rather than merely a de facto one (like the Roman Empire under the Caesars), today might be a holiday. For today is our king's birthday. President Donald Trump beat me into the world by almost two years at the very beginning of the "Baby Boom." Happy 71st Birthday, Mr. President! Since Marilyn Monroe is lamentably no longer with us, we will have to make do with this old recording from May 1962: https://www.youtube.com/watch?v=EqolSvoWNck.
By convenient coincidence, today is also Flag Day, the official birthday of "Old Glory." As we all learned in grade school, on June 14, 1777, the Second Continental Congress passed the Flag Act, which declared that the new flag would have "13 stripes alternate red and white: that the union be thirteen stars, white in a blue field, representing a new constellation." At 240, the flag trumps all of us aging Baby boomers in seniority! If we ignore the changes over the years in the number of stars, the US Flag is surely one of the oldest national flags continuously in use. Even the current UK "Union Flag" dates back only to the creation of the United Kingdom in 1801, when the Irish Cross of St. Patrick was added to the already existing combination of England's Cross of St. George and Scotland's Cross of St. Andrew. But, if we treat that change analogously to the addition of stars to the US flag, then, of course, the British "Union Flag" - uniting the Crosses of England and Scotland and dating back to 1606 - is older than the US flag. (That British flag was obviously familiar to the American founders and presumably provided the American emblem with its famous red,white, and blue color scheme.)
For all the attention the flag gets in American popular culture, however, Flag Day has never managed to make it into the pantheon of official federal holidays (of which there are 10). So the day is acknowledged or not, according to individual and local priorities. (It has, however, been a state holiday in Pennsylvania since 1937.)
According to https://www.whitehouse.gov/briefing-room/presidential-actions/proclamations. as of May 31 President Trump has proclaimed the month of June 2017 "National Homeownership Month," "National Ocean Month," "African-American Music Appreciation Month," "National Caribbean-American heritage Month," and, last but not least, "Great Outdoors Month." Given the role devotion to the flag has often had in our popular culture (obviously a symbolic substitute for a monarch as a focus of national loyalty and symbol of unity), perhaps a presidential proclamation of a "National Flag Appreciation Month:" might be in order.
Or, even better, a full-fledged federal holiday, honoring both the king's birthday and the flag's?