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This is the last project we need to do before graduation. It's definitely not the easiest one. We have been working on it [day after day/day in and day out].

Which is correct?
Thanks.
+0
This is the last project we need to do before graduation. It's definitely not the easiest one. We have been working on it [day after day/day in and day out].
Comments  
Could you explain the difference?