Sentence:  ... a rainy Palm Sunday. ... At Grace Church on-the-Hill, the children and the acolytes stood huddled in the narthex; holding their palm fronds, they resembled tourists who'd landed in the tropics on an unseasonably cold day. The organist chose Brahms for the <WSD>processional</WSD>—“"O Welt ich muss dich lassen"”; “O world I must leave you.” 
Meaning of the word marked with <WSD> tags:  A hymn or other music used during a procession; prosodion.