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1st January 12PM or before and lasts to 2nd (this is national hangover day), 3rd, or if its really bad, 4th January (these are local hangover days). Perfect day to do crimes, because nobody cares. But the entire country will think the robber is a loser because he doesnt have hangover.
Person 1: Hey, wanna do something funny 1st January?
Person 2: Dude, its National Hangover Day
Person 1: Oh yeah, right ..
by Jellies January 10, 2007
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4
The day following any popular holiday.

Including but not limited to the day after:
New Years Eve (New Years Day)
The Super Bowl
Memorial Day
Fourth of July
Labor Day
Thanksgiving (Black Friday)
Christmas (Boxing Day)
Girl: I'm not getting any work done today, I'm just wasting time.
Boy: It's okay, it's National Hangover Day, a lot of people are wasting time.
by Fun Johnny February 07, 2011
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5
December 26th, the day after Christmas. The day where nobody has the energy or will power to get there hung over asses out of bed after a hard night of drinking, present opening, face stuffing, fucking (if your lucky), disapointment, and in some cases humiliation and shame. A day that is usually celebrated by staying in bed till 4 and finally getting up only to spend the rest of the day vomiting your guts out.
Guy1: Christmas is gonna be awsome this year!!!! Guy2: Hell yeah!!!! But fuck man, I sure ain't looking forward to National Hangover Day... Guy1: Fuck it, we'll just be in bed all day anyways, so no need to worry about it.
by Xero _ Manifest December 25, 2010
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6
The day after all the New Year's parties and celebrations. People are usual too hungover to begin turning around there lives like they promised themselves just a couple of days ago.
Mark: Jake wake up! You have rent due, and your gym membership starts today!

Jake: ugh, but its national hangover day.. *vomits*

Mark: Goddamn it Jake.
by sladyconesmom December 31, 2014
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