Three days after the backing track for Because was recorded, The Beatles added the song's distinctive vocal harmonies.
Having done the backing track, John, Paul and George sang the harmony. Then we overlaid it twice more, making nine-part harmony altogether, three voices recorded three times. I was literally telling them what notes to sing.
The Complete Beatles Recording Sessions, Mark Lewisohn
The vocal recording session took place in Studio Two from 2.30-9pm. From 7.15-8.45pm a brief mixing session was held in the control room of Studio Three. Harrison oversaw the creation of rough stereo mixes of Something and Here Comes The Sun, to allow George Martin to prepare orchestral scores.