In early 1939 England war was clearly brewing with rampant fascism across Europe, the UK included. 
As a homosexual Jewish pacifist with distinct leftist leanings, Britten had every reason to feel vulnerable. Also he was being pilloried by some critics – and he was always
hypersensitive to criticism.
He had also just met 
Peter Pears, who would become his life-time’s partner, and he had urgent need to disentangle himself from an assortment of ‘romantic attachments’. So in April Britten and Pears left for Canada.
They soon moved on to Woodstock where Aaron Copland was living, spending the summer there; 
but they settled more permanently at Pears’ friend Elizabeth Mayer’s house.
Mrs. Mayer became something of a mother to Britten, and he dedicated A Ceremony of Carols to her.
Towards the end of his time in USA,
Britten was planning to write a harp concerto. He had therefore been studying the instrument in depth and was now fully primed for composing A Ceremony of Carols. At this time he also composed A Hymn to St.Cecilia.
In March 1942 Britten and Pears set sail from New York, in the Swedish merchant ship Axel Johnson. Passing through emigration the officials confiscated the scores for the Harp Concerto and Hymn to St. Cecilia, thinking they contained subversive messages in code. On the first leg of the voyage, Britten rewrote Hymn to St. Cecilia.
Since America had joined the war after the attack on Pearl Harbour, the eastern seaboard of USA was heavily targeted by German U-boats and many allied vessels were torpedoed during the course of the war. They must have undertaken the crossing with much trepidation.
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, The English Galaxy of Shorter Poems. During the Atlantic crossing he set 7 of these for boys’ voices and harp. These he expanded to give us ‘A Ceremony of Carols’, completed in 1943.