Unless you're Egyptian or Sudanese, or have hung out in one of those countries, or are an Arab who's watched a lot of Egyptian movies, you may not know about the holiday celebrated today. Yet arguably it may be the oldest holiday celebrated anywhere, and its name may preserve an ancient Egyptian name.
Sham al-Nassim is Arabic, and the words mean "smelling the air," or "smelling the breezes" if you prefer. Other than the specifically patriotic days, such as the National Day, Military Day, etc., it's the only Egyptian holiday celebrated with equal ardor by Muslims and Copts, and by Jews when Egypt's Jewish population was significant. For the past couple of thousand years, it has been celebrated on the Monday after Coptic Easter (which coincides with the general Eastern date for Easter), thus today.
Egyptians of all religious identities get the day off and picnic along the Nile, if they live near it, or go to parks if they don't, eat a dried fish called fassikh and several other traditional spring treats (though my memories of fassikh are not all that endearing: just dry, salty fish), and generally "smell the air" of spring. (According to this site, they also paint eggs. I don't recall seeing that, and perhaps it's a Western Easter import, or I just missed it.)
That's just for the past couple of millennia, though. Wikipedia's article notes the purported link to the ancient Egyptian feast of Shemu, "creation" or "new life," celebrated at the spring equinox, which has been documented (at least according to Wikipedia and its Egyptian source) to 2700 BC in the Third Dynasty. On the other hand, Wikipedia's separate "Shemu" article suggests it was a movable feast in the dry (low Nile) season. Presumably the feast shifted with the advent of Christianity to coincide closely with Coptic Easter, but remained essentially a spring equinoctial celebration. Since the Muslim calendar is purely lunar, the holiday stayed, like some others (including the Nile flood holiday), linked to the Coptic calendar. Somewhere I believe I've read that Plutarch even mentions the Egyptians eating dried fish at the equinox: if so, fassikh has been around a while. (On the other hand I tracked down a reference to this Plutarch statement, and it was Wikipedia citing the Egyptian State Information Service, and the State Information Service link just goes to the main page. So the scholarship here may be a little edgy. I'm no Egyptologist: I start with the Arab conquest and come down from there.) (Egyptology/Coptology/Late Antiquity Grad students: term paper subject? Send me your results.)
Like Easter itself, which in English at least combines a Christian feast central to Christian belief, but based on the date of Jewish Passover, with a Germanic word relating to fertility (compare "estrus"), Sham al-Nassim is a historical palimpsest, a syncretistic hodgepodge, that has — besides being a great spring holiday for Egyptians of all faiths or none — finally given me the rare opportunity to use "palimpsest," "syncretistic," and "hodgepodge" all in the same sentence. (Class, you may use your dictionaries.) The ancient Egyptian spring festival was first baptized by placing it on Coptic Easter Monday, then Islamized or at least Egyptized by being adopted as the spring holiday for everybody regardless of religion.
I think the only other ancient Egyptian feast that survives in the Egyptian calendar today may be the Wafa' al-Nil in August, celebrating the Nile flood and also retaining elements of pagan, Christian and Muslim eras, but fading a bit I think since the end of the annual flood with the building of the Aswan High Dam.