I keep trying to find a player comparison for Day. It's tough to do. He's very much original in his style.
From a pure skating standpoint he's Nick Leddy. Smooth and effortless, yet can really fly! But he doesn't defend like Nick Leddy does.
As a player I think he reminds also of Vladimir Malakhov. Malakov had off the charts skills, and he was also a giant of a man. If you watched him in practice, you would think he by far was the best player on the ice on any given night. He could stick handle with the best of them. Strip the puck off the best forwards of the time with a quick poke check like he wasn't even trying. His skating was just effortless, and for such a big dude it was so uncommon. Could lay people out with a big hit, or rub people out along the boards, but then could let guys fly past him unimpeeded. At the time the big lumberjacks of the NHL skated in sand for the most part. Malakhov didnt. But then game time would come, and he never really stood out anymore. Like it was if he didn't care if his club won or lost. He never had the extra effort. If he did, he certainly didn't show it. He should have been among the best defenseman in the game of his era. He, however will never be remembered as such.
In Day, skill level is off the charts. He has that air about him that he could be the best player on the ice at any given night. But then, for whatever reason he doesn't factor into games like his size, skill, speed and presence suggest.
Rangers should send him into a Tony Robbins seminar, followed up by a week with Adam Graves mentoring.