Now, why did DRA have to become president for a day?
Well, it's disputable, at best.
But, basically, here it is:
James K. Polk's term ended on March 4th as usual. However, that year, the date fell on a Sunday, the Sabbath. His successor, Zachary Taylor, was devoutly religious, and refused to take an "Oath to God" on a Sunday. Both he and his VP, Millard Filmore, were sworn in on the next day, the 5th. However, this was before the current notion that the President-elect's term begins on Inauguration day whether or not he takes the Oath.
David Rice Atchinson, who was President pro tempore of the Senate, was next in the line of succession. However, the Senate's previous term had ended on the 3rd, and didn't reconvene until the 5th (I think). In the time between, there was no PPT, so Atchison wasn't really in the line of succession.
Anyway, everyone (then and now) considers Taylor's term to have commenced on the 4th, so the point is null.