What they do is track when their listing count is set to hit it's minimum. Then they do their 7 day free listing day 7 days before that day.
Because that dip happens on a different day each year they wait until late december to plot it out and then decide when to have the free listing day.
They usually announce it somewhere around 5pm-7pm the day before.
Here is where you can check:
http://www2.ebay.com/aw/marketing.shtml
They'll post there once they decide so if you checked daily at 8 or 9pm or first thing in the morning you'll know.
These are just my own opinions of how they do it but I've been around for 5 years of these or more and that seems to be how it goes.
My guess is for the 27th or 28th, or the 22nd or 23rd