。
Tomorrow never comes.
Literal
Tomorrow [quotative-と] say day [topic-は] not-come.
A proverbial phrasing. 明日という日 = 'the day called tomorrow' — using quotative と + 言う + noun ('the X called Y') to attach a descriptive label. Philosophical/admonitory in tone: tomorrow, as a day, never actually arrives, because when it does it becomes today. A 'carpe diem' sentiment.