I have read about various battles marking the end of the "era of the one-day battle". Which battle holds this distinction? Or is the whole idea of the "era of the one-day battle", a time in military history when battles longer than a day were very rare, a load of crap?
Do you consider sieges to be battles?
I assume you mean without a pause in fighting for the night. If so, then the "era of one-day battles" is definitely a thing.
I'm going to re-echo what another poster has already asked. Do you mean over several continuous days with major fighting occurring even at night? Or do you mean battles in which most of the fighting was restricted to the day, with only skirmishing at most during the night? If the former, such battles are still incredibly rare, and I don't know of any before the 20th Century. If you mean the latter such battles have been fought for a long time. Artemisium lasted three days, as did Thermopylae. One could argue that Plataea took about a week, as opening engagements were taking place the entire time the Greek force was mustering, long before they marched forward to begin the general engagement. Philippi lasted about two weeks, although there were only two periods of major engagement (Syme refers to it as a double battle)