It think it only became set on the 25 of May after he was diagnosed with Alzheimer's. In the book themselves the date is never mentioned and, in fact, no flat-world dates or years are used.
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.
I believe that when Pratchett adopted the Lilac for raising funds for Alzheimer's research that is worn on the 24 of May.
This mention has led, since Terry Pratchett's embuggerance to the flower's adoption as the symbol of support for Alzheimer's research fundraising.
Terry Pratchett made a donation of $1 million to the Alzheimer's Research Trust and has begun campaigning for more research and awareness. "Match It For Pratchett" [now gone, Wayback Machine link] was begun by fans in 2008 to match his donation of $1 million dollars by selling bunches of fake lilacs, as well as stamps, jewellery and even towels (to tie in to Towel Day, which is also on May 25th). The original site is gone, but Wear the Lilac day is alive and well!
This site mentions Towel Day:
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.
So it would seem that the date to wear a lilac in support of Alzheimer's has been taken as the date of the Glorious Revolution by all but... as far as I know there is not date in the book.