I pretty sure that fresh wort has tons of microbes in it. If you left it for 24 hours it would sour. People do this intentionally so the sour taste transfers into the final product. Obviously boiling would eliminate the souring of the microbes as they would be killed by the temperature of the boil. Best bet is to have everything ready the 1st day. Second day Mash, boil, and just chill the wort over night in the fermenter. Pitch yeast the next day and clean up.