Green’s Dictionary of Slang

any frequently served dish, e.g. ham and eggs, bacon and eggs, mutton.

1921		T.A. Dorgan in Zwilling TAD Lex. (1993) 133: We call mutton ‘365’ because we get it every day of the year.
1959		I. & P. Opie Lore and Lang. of Schoolchildren (1977) 183: Rice pudding, says a Manchester boy, is ‘Three-six-five pudding’ (’because it is served every day’).
2021		Betoota-isms 98: 365ers [...] 1. Mutton chops, because farmers will eat them 365 days a year and never get sick of them.