There was a topic on boards.wizards.com. Not anymore, alas, they keep on deleting old threads. Who can we trust ?
The thing is, drows are chaotic. They just can't do something the same way all other people do. So, a "year" for the dark elves is not a year in the way we understand it. First, years are determined by the cycle of seasons, the sun or the moon. None of these exist underground. How could them use normal years, then ? They don't. Instead, a "year" is simply the duration of a matron mother's rule. When she's replaced by another, it's a new year. There's a New Year celebration, where every dark elf makes "good resolutions" (oath of fealty to the new matron in public, oath to kill her in an inventive way in secret). Then the new matron declares the theme of her reign (=year). Being vainglorious creatures, this theme is invariably "Year of the Drow", "Year of the Dark Elves", "Year of the Spider-Kisser", "Year of the SM Lesbian Boink Bunny", "Year of the Dual-Scimitarred Good Hearted Renegade", and other concepts associated to darkelvendom.
Since the days of Lorraine Williams, there's a tradition in TSR, and now in WotC, of having an evil matron mother ruling secretly over the company. Everything is now explained.