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1.
The first day--usually a couple of weeks after the start of spring in areas which experience significant seasonal variation (such as the northeast US)--on which a large majority of girls and women wear their newest skirts. This occurs en masse, leading one to blurt, "Oh, thank God for Skirt Day." Skirt Day generally occurs on the second or third day of consecutive, skirt-encouraging weather in a week (at least mid-60s, winds < 10-15mph).
I woke up this morning, strode outside, saw dozens of beautiful girls in skirts on the way to class, said a brief thank-you to God for their smooth and fit legs, and promptly declared to my friends, "Today ... is Skirt Day."
by Rarathelunchman April 04, 2009
 
2.
Skirt Day - (n.) - The spring day that is just warm enough, predictably in the 65-70 degree range, where every girl's biological alarm clock chimes. The young Co-Ed's will don their floppy skirts, uggs or flipflops (or uggflops if they existed), popped pastel polos of various vintages & fashion shops as well as sunglasses with fist sized lens. Precursors include but aren't limited to long lines at Campus Tan, overcrowded ellipticals at the plex, and sudden sell-outs of Crest White strips and the Proactive System...
On April 10, 2008 the campus of Boston College celebrated Skirt Day.
by N Cam April 09, 2008