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It's just the way this particular expression is used. Don't try to analyze it too much. It's not something that will apply to other expressions. Actually, it sounds a little old-fashioned to me. (I'm American.)
Come fall (autumn), these little plants will be several feet high.
Come September, he'll be leaving for college.
The Following User Says Thank You to vivemafille For This Useful Post:
...come + time 
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