Paul McCartney is probably the greatest songwriter of all time, perhaps behind only Irving Berlin. Granted, he partnered often with John Lennon, but McCartney's approach had the golden touch, and he's been a songwriting icon for the past 60 years.
Paul McCartney was a lover of classical music and drew much inspiration from those classics. I don't mean that he copied them, for example: Eleanor Rigby can easily be declared a songwriting masterpiece. When McCartney was crafting this mythological epic, he was listening to Vivaldiās āFour Seasonsā on heavy rotation.
For the string arrangement that backed the song, McCartney asked George Martin to craft him something based on Vivaldiās classic movement. Whilst the end result only shares a vague likeness with the Winter section of the movement, the scope and dalliance remain the same, ultimately producing a brooding piece of pioneering pop-classical cross-over.
Similarly the grand scope and ambition of Sgt. Pepperās is epitomised by the meandering epic that is āA Day in the Lifeā. The song works its way through a journey, much in the same way that a classic piece transitions through āmovementsā.
For āA Day in the Lifeā, George Martin was tasked with permeating the song with epic gathering crescendos. To create these chaotic climaxes, he gathered an orchestra and tasked them with improvising.
Around this same time, Witold LutosÅawskiās was causing a stir in the classical world for applying a very similar loosely improvisational technique, and there are elements of his work evident in the basic structure that Martin wrote for the orchestra to work around.