Bookmark this link and keep checking for 'Zero insertion fees'. (You'll see that the dates are only announced just one day in advance):
http://www2.ebay.com/aw/marketing-uk.shtml
smurf, I get an email from eBay when free listing is coming up (usually on the Friday evening before it happens) - it was free listing for me last weekend (over the bank holiday) so it probably won't be this one, more likely the weekend of 7-8 September. It's always shown on their Announcements on the bottom right of your eBay page.