The AT has certain celebratory days, most of them are location based, not based on a calendar. Most are common to all thru hikers, yet on different dates. The big 4 I can come up with is:

Start of hike (for many that is what I call 'Springer Mountain Day')
1/2 Gallon Challenge
Hike Naked Day (the only one that is calendar based)
Katahdin (or end of thru hike)

Of the above 4 the 1/2 gallon challenge date will most likely fade from my memory as to the date (though if I revisit the location that's another story) but unquestionably a AT 'holiday', the other 3 I believe I will always celebrate.

So what are you AT holidays?