Lennon wrote the song, not quite word for word from an old 19th century circus poster that he purchased in an antique shop, while filming the promotional video for the song "Strawberry Fields Forever" in Kent.
The poster begins:
"PABLO FANQUE'S CIRCUS ROYAL,
TOWN-MEADOWS, ROCHDALE.
Grandest Night of the Season!
AND POSITIVELY THE
LAST NIGHT BUT THREE!
BEING FOR THE
BENEFIT OF MR.KITE,
(LATE OF WELLS'S CIRCUS) AND
MR. J. HENDERSON,
THE CELEBRATED SOMERSET THROWER!
WIRE DANCER, VAULTER, RIDER, &c.
On TUESDAY Evening, February 14th, 1843.
Mssrs. KITE and HENDERSON, in announcing the following Entertainments ensure (sic) the Public that this Night's Production will be one of the most splendid ever produced in this Town, having been some days in preparation."
The poster also mentions "The Celebrated Horse Zanthus" (not Henry), contains several references to the Hendersons, and advertises Mr. Henderson's "TRAMPOLINE LEAPS AND SOMERSETS! Over Men & Horses, through Hoops, over Garters and lastly through a Hogshead of REAL FIRE!"
Lennon wanted the track to have a "carnival atmosphere" - he is even said to have told producer George Martin that he wanted the song to smell like sawdust. In the middle eight bars, multiple recordings of pipe organ, calliope, and harmonium music were edited together to attempt to produce this request, and the result was then added to the backing track.
Later in his career, Lennon said that this was one of his least favourite songs.
External Link
Discussion of the original poster, with images and text