The day following a hard Friday (day or night) of drinking or other forms of debauchery, and the participants are looking and feeling haggard. The day preceding Sunday (only if previous conditions apply).
"If we drink too much, tomorrow might make for yet another Haggarday."

"Man, I got way too drunk last night. I'm not looking forward to this Haggarday."
by Javier7 September 6, 2008
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