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1.
A phrase used to describe a stark difference between two things. Similar to day and night, except it implies an improvement of the situation rather than a deterioration.
Democrat: "Wow, I'm so happy that Obama is in the White House now. The difference in our country's leadership has been night and day."
Republican: "Actually it's been day and night. George W. Bush was the man."
Independent: "You're both wrong. It's been night and night. Both parties are corrupt as hell."
Libertarian: "No, I'd say it's been more of dusk and twilight."
Left-leaning moderate: "You're crazy. It's totally been dusk and dawn."
Normal person: "Seriously guys, shut up."
by Nicholas D March 15, 2009
 
2.
a phrase use to denote different or far from. 'Day and night' conveys a similar meaning.
1. She is a bluestocking and night and day (=different) from her contemporaries.
2. What she has achieved so far in her studies is night and day from (=far from) satisfactory.
by uttam maharjan November 23, 2011