night and day
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night and day
1. All the time; continuously. I've been studying night and day to get ready for this test.
2. Describing a contrast between two completely different things, often one that has resulted in improvement. Often preceded by "like." Her transformation has really been night and day. She doesn't even look like the same person. A: "Have you seen any improvement in his behavior?" B: "Oh, it's like night and day. He's been a model student this month."
night and day
Also day and night. Continually, without stopping. This phrase is used either literally, as in The alarm is on night and day, or hyperbolically, as in We were working day and night on these drawings. Shakespeare put it by night and day in The Comedy of Errors (4:2): "Time comes stealing on by night and day."