Ok, so maybe not a day. The erythrocytic part of the life cycle (which is responsible for the cyclic paroxysms as previously mentioned) takes more like 36 hours, or 48 or 76 depending on which species it is.
Whichever one you look at, the process if not the timing is essentially the same; when an infected mosquito bites a human it injects saliva. This contains chemicals that act as vaso-dilatators, anti-coagulants and anti-haemostatics, which are all necessary for the mosquito to have a successful blood meal. However if the mosquito is infected with Plasmodium, it is likely to be carrying Plasmodium sporozoites in its salivary glands. These are injected along with the other chemicals in the saliva, and then travel to the liver, where they reproduce asexually and enter the bloodstream. The sporozoites enter red blood cells and again reproduce asexually, eventually causing the red blood cells to rupture releasing merozoites. This process is repeated exponentially, as every time a red blood cell bursts it infects others, destroying large numbers of red blood cells and resulting in the typical symptoms of malaria sufferers; anaemia and paroxysms (chills and fever). In some cases the merozoites will develop into male and female gametocytes inside the red blood cells, which in turn can be taken up in a blood meal by the female Anopheles when feeding on an infected human host. The mosquito is an essential part of the Plasmodium life cycle as the gut of the mosquito is where the gametocytes fuse, producing ookinetes which further develop into oocysts and then finally to sporozoites, which migrate to the salivary glands ready to start the process all over again when the mosquito finds its next victim to feast on.
For more information on feasting mozzies:
malariamadness 