I am 100% certain you could do it the day of before. Next to the players name the only option was "arbitrate" on the day of (but the day before arbitration there was a second option for long term deal). But before, on the day of arbitration, there was a couple of paragraphs on the arbitration page explaining how arbitration works and within those paragraphs was a link to negotiate long term. Those paragraphs, and the link, are now gone. And so, it would appear, is my Ace relief pitcher after this season! ??