Some people find they work best when under pressure. If you're doing it the day before, you really do need to devote the entire evening to it. Some essays can be written with a certain degree of 'blagging it'... You can write about something based upon previous knowledge, or just give your own interpretation on something, as long as it's on topic. Throw in a few critics' views, a conclusion, keep going until you get reasonably close to the word count and Bob's your uncle. In my experience, though I wouldn't advise it, this tends to work alright with some English essays, as long as they're phrased well and ideas link onto each other.
For a subject like History though, I agree when you say it's important to 'read around' the subject first if you're not already familiar with it. If you haven't got the chronology sorted in your mind, it's pretty difficult to analyse or create a coherent argument.