I think I have posted a link to this music before, but anyway. I will always associate this with Anzac Day: Vaughan Williams was an English composer who served as a stretcher bearer/medic on the Western Front and in Salonika during World War One.
He composed this after the war. It is shot through with grief, with an awful, haunting sense not just of loss and of waste but of something irretrievably broken.
There is a very good write-up about this here, even if some of the more technical musical stuff goes over my head.