Composer: Jeremiah Clarke
Trumpet Voluntary (more correctly known as the Prince of Denmark's March) is one of the more popular choices for classical wedding processional and recessional music. It comes from English Baroque composer Jeremiah Clarke. Princess Diana walked in to it at her wedding in 1981. Clarke's other often-chosen work for weddings is Trumpet Tune (also a royal wedding favorite).
While it is most commonly associated with weddings, it has been heard in a number of other settings, including BBC broadcasts during World War II - particularly when the programming reached occupied Denmark. The Beatles made use of it during the final fade in their song It's All Too Much. And Sting made it a melodic counterpart in his song All This Time. The Colbert Report uses it regularly on a couple of recurring segments of the show (Colbert Platinum - with trumpet, and Colbert Aluminum - with kazoo). The Royal Clock in the Queen Victoria Building in Sydney, Australia plays it during the clock's hourly performances.
As with Trumpet Tune, Clarke's Trumpet Voluntary is sometimes erroneously attributed to the other famous English Baroque composer Henry Purcell).
It is primarily performed by organ (not by trumpet as you might assume), but can certainly be adapted for trumpet soloist and piano or organ accompaniment if you prefer.