。
Day after day, I wrote her letters.
Literal
Come day [also-も] come day [also-も] I [topic-は] she [to-に] letter [object-を] wrote.
来る日も来る日も is a literary set phrase meaning 'day after day / day in, day out,' built from repeated 来る日 ('coming day') + も — the repetition with も emphasizes monotonous persistence. 手紙を書く ('write a letter') is a standard collocation, with に marking 彼女 as the recipient.