First, to maintain the similarity to upcoming HPMoR release dates; saying "this is the final chapter of this story, but the next text released by Eliezer comes out at 10:00 AM Pacific Time on March 15th, and is a nonfiction work explaining the actual methods of rationality" seems like it will manage to capture many HPMoR readers whose attention Eliezer might not have again. I suspect that a day-long wait before release does a good job of accruing benefits from anticipation without paying too many costs from exhaustion or attention loss.

Second, I assume that it will be easier to release it on the 15th than the 14th, and the 14th will be easier than the 13th, and so on.