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On Monday. I don't have enough money right now, but don't worry, I'll pay you back come Monday.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
Rur. when Monday comes. (Can be used with other expressions for time, as in come next week, come December, come five o'clock. See the second example.) Joe plays so hard on the weekend that come Monday, he's all worn out. You may think that putting up storm windows is a bother, but come December, you'll be glad you did it.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.