Britten composed A Ceremony of Carols while crossing the North Atlantic in 1942 – in convoy HX183. He and his partner Peter Pears had been in North America since April 1939; but when America declared war after Pearl Harbor they felt they must leave for home, setting sail from New York in March 1942, in the Swedish merchant ship Axel Johnson, for the perilous return journey.
At that time the North Atlantic was being patrolled by German U-boats (3,500 allied merchant vessels were lost crossing the Atlantic). Because of this threat, merchant vessels sailed in convoy, escorted by warships.
So the Axel Johnson sailed first to Halifax, Nova Scotia to join its convoy. While waiting in Halifax, Britten found an anthology of medieval verse in a bookshop: The English Galaxy of Shorter Poems. He set 5 of the poems, plus 2 extra, for boys’ voices and harp during the Atlantic Crossing (two of the settings have surprisingly war-like overtones). These he later expanded to give us A Ceremony of Carols, which he completed in 1943.
The Atlantic crossing was calm and uneventful.