You get many now, often young people who turn up at stations and find them empty (and then rant on social media)If anything I think this plan could do with less publicity. If people just see the headline "Northern to run trains on Boxing Day" in their local paper (or website), they'll assume that trains are running on all lines and will be in for a shock when they turn up at their local station to find it deserted!
Best keep it (relatively) quiet and, if it goes well, it will be a good indication of whether to run more extensive Boxing Day services in future years.
Especially in places like Leeds who now have quite a comprehensive bus service on Boxing Day, so they’d expect trains to be running as well.