Question

why do we prepare idli or Dosa dough one day before

Answer 1

this is done to make the dough fluffy .  The dough becomes fluffy by the action of certain microbes and trapping of CO[tex] 2 [/tex].
this process can be fastened by keeping the dough in the sunlight for a few hours

Answer 2

to make the dough fluffy by trapping of microbes n co2 the yeast is formed in the dough