Naturally there are some terms and conditions attached (they're here) which amount to "don't be silly." Here's the pitch from Vodafone:
It's easy. Go online on your phone all day Friday 27 November and mobile internet won’t cost you a thing, all day long. This offer is open to everyone on Vodafone, whether you're on pay as you go or you’re a pay monthly customer. And if you have a mobile internet bundle, the data you use on Free Mobile Internet Friday will not count towards your monthly data allowance.
Full details on the offer can be found here. I wonder if this is going to be a one-off?