From Discworld & Terry Pratchett Wiki
Truth, Justice, Freedom, Reasonably Priced Love, and a Hard-Boiled Egg!
The People's Revolution of the Glorious Twenty-Fifth of May ended the increasingly tough reign of Lord Winder. Tension had been rising, and while the nobility arranged a quiet succession by Lord Snapcase in the background, the people on the streets started a revolution and attacked Watch Houses all over the city.
A few streets around Treacle Mine Road were barricaded at first. Soon more people started barricading streets, barricades were moved forward and merged together, covering at least a quarter of the city - including the food industry. The resulting area was called The People's Republic of Treacle Mine Road. The watchmen of the Treacle Mine Road Watch House led the Republic together with some enthusiastic angry young men, among them the then-living Reg Shoe.
Although the Republic originally was invaded succesfully, the change of history recorded in Night Watch resulted in Sam Vimes under the name of John Keel saving the Republic until Lord Snapcase had become Patrician. But because history finds a way, those who died in the original revolution still died, in an attack planned by Carcer prompted by Snapcase's concerns about what "Keel" could get up to if left alone for a month after serving as such a prominent leader after less than a week in the city.
Each year, on the 25th of May, a group of survivors of the uprising gathers at Small Gods' Cemetery to honor the casualties with lilacs and, affectionately, one hard-boiled egg (from Madam Roberta Meserole). The seven killed were mostly Watchmen from Treacle Mine Road : John Keel, Cecil Clapman, Horace Nancyball, Billy Wiglet, Dai Dickins, Ned Coates, and, temporarily, Reg Shoe - he will lie in his grave for a time during that day, and then leave.
The 25th of May is also memorialized, among those who survive, by the wearing of lilac on that date. Persons known to wear it include Sam Vimes, Fred Colon, Nobby Nobbs, Cut-Me-Own-Throat Dibbler, and, improbably, Havelock Vetinari (he, at the time a young assassin, has kept his and his aristocratic aunt Lady Roberta Meserole's, not-insignificant involvement in the affair entirely secret).
The date is not publicly known as it was one of those revolutions where everybody likes to pretend in the aftermath that it never happened, with many new Watchmen uncertain of its relevance to the point that one new recruit tried wearing lilac only to be sharply criticised by Fred Colon. Vetinari once speculated about erecting a statue in memory of the soldiers, but Vimes rejected the idea, stating that the dead men would not want to be immortalised and inspire others to be heroes after they were betrayed for going beyond the call of duty, requesting that the men be simply left in peace.
Following Terry's announcement about Alzheimer, calls have been made to wear lilac on the 25th of May as a tribute, and to raise money for Alzheimer research.
In the Earth history the name "Glorious Revolution" is usually ascribed to the overthrow of King James II of England by the Dutch invasion force under William of Orange in 1688.
It is noted that in Earth history the 25th Of May was national holiday in former Socialist Yugoslavia as it was birthday of Yugoslav communist leader and WWII resistance leader Josip Broz- Tito. Holiday was also known as "Day of youth". It is also a day in 1944 when German paratroopers tried and failed to capture him.
Also, the 25th of May of 1810 is recorded in Argentina as the start of the Revolution against Spain, the "Revolución de Mayo" ("May Revolution", in english), centered around Buenos Aires, but pretty much igniting the revolution fires across Latin America.
In WWII in Yugoslavia many partisan-held territories were called "Republics" or "Peoples Republics"
May 25th is also national Geek Pride Day and Towel Day, a day in honour of Douglas Adams. This has led to some fans having to choose between the two, until someone came up with the lilac towel.