Today is the anniversary of the founding of Rome - in other words, Rome's birthday.
April 21 is sacred to the the goddess Pales, and Rome therefore celebrated the Palilia or Parilia, an agricultural/pastoral festival. Although this festival probably pre-dates the founding of Rome, the ethos of what it meant to be "Roman" was so wrapped up in landowning and farming that it's likely the date was specifically repurposed for the birthday of Rome.
The year of Rome's "birth" is also uncertain; during the Empire, Varro suggested 753 BC but the Fasti Capitolini say 752 BC. So I'd say that this is Rome's 2,765th birthday, but others would disagree.
Back in 2007, I was in Rome for her birthday, along with my husband (who snapped these pictures and video) and former student Lara. The start of the parade looks (and sounds) like this:
And here's a slideshow of some of the awesome costumes on display: