"Tomorrow never comes" is a well-known English phrase. Strictly speaking, I think it means that "tomorrow" will never come, because when it's "tomorrow", that day will then be "today", and "tomorrow" will be the day after that. I think it's used as a reply to "I'll do it tomorrow", to jokingly point out that "tomorrow" is a kind of an illusion, and that saying "I'll do it tomorrow" means that you might never do it.
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