Of Ngāti Tūwharetoa descent, Day was born in Palmerston North and also lived in Christchurch before moving to Wellington when she was 14 years old.

She was a stay-at-home mum for 10 years until her youngest child started school, when she re-entered the workforce with a part-time job helping children who were falling behind with literacy skills.

[1] Day felt urged to run for the Wellington City Council after meeting a homeless family in 2016.

[4] She held this position until 2019 when Lester lost the mayoral election to Andy Foster.

[10] In July 2022, she was elected unopposed to be the next president of the Labour Party after Claire Szabó stated she would not seek re-election that year.