To meet the print deadline for The Herald's Saturday Arts, I am writing this on Unesco International Jazz Day, Thursday April 30.

This is when Glasgow Unesco City of Music showcases school jazz bands in the afternoon and then a triple bill of professional outfits - Ken Mathieson's Classic Jazz Orchestra, duo Fraser Fifield and Graeme Stephen, and quartet Brass Jaw - in the Royal Concert Hall building, as well as providing a platform for the launch of the programme for this year's Glasgow Jazz Festival, which runs from June 24-28.

When Glasgow Unesco City of Music director Svend Brown and I discussed the matter, it was a mystery to us both why the United Nations Educational, Scientific and Cultural Organisation had, as recently as 2011, decided that April 30 was the day for the world to celebrate the great music of the 20th century. When you consider that the previous day, April 29, is the birthday of the first great composer of the music, Duke Ellington, born that day in the last year of the 19th century, it begins to look like a bizarre scheduling error.

However, the On This Day column at the foot of Thursday's Herald letters page suggested a possible historically rigorous reason for the choice. Almost a century earlier, on April 30 1803, the US bought Louisiana and New Orleans from France for fifty million francs. The French origins of the cradle of jazz music are crucial, because it was French settlers who brought brass instruments over to New World, and with them the French tradition that every small habitation has its own band. Go to a Murrayfield rugby international against Les Bleus, and you may be sure that there will be a small marching band amongst the travelling support.

During the 19th century, many of the French people who had come to seek their fortune/exploit the resources returned home, while others were absorbed into the new American nation, sometimes finding themselves in reduced circumstances. As a result the pawnshops of New Orleans were filled with unwanted clarinets, trumpets and saxophones. Those previously-owned instruments were acquired by African Americans, and teamed with the guitars and pianos on which the blues was already played, in the creation of jazz music. It is a wonder there isn't a verse or two that explains this for Bing to sing in High Society, so Unesco are presumably just making up for that omission in the 1956 movie.

You may chose to believe my version of musical history or not, but what is beyond argument is the status of Glasgow Jazz Festival as the longest running arts event, never mind musical one, in the Dear Green Place. Next year will be the jazz festival's 30th birthday and this year's programme has concerts across the city, from Gladys Knight and Frank Sinatra Jnr at the Concert Hall, via The Family Stone at the O2 ABC, to two of Scotland finest young pianists, Pete Johnstone and Fergus McCreadie, at Cottier's in the West End.

The core of the programme can be found in the Merchant City, with venues from one end of Candleriggs (the Ramshorn) to the other (Wild Cabaret and the Tron Theatre). In the middle of Jazz Street are the two venues the festival has created, the Old Fruitmarket, rescued from its status as a car park in the 1990s, and the Rio Club, the more recent re-purposing of a basement at Merchant Square, which has the ad hoc feel of the former's early days. At the end of June, the jam there goes on until the small hours - so get your horn out the pawn.

www.jazzfest.co.uk