Apologies if this is explained somewhere already, but I have two questions here. 1. Why is the genitive case used here? 2. Why is it genitive singular and not plural if we are speaking of 2 days?
The numbers 2, 3 and 4 require genitive singular in russian. Afterwards it's genitive plural. Два/три/четыре дня, пять/шесть и т.д. дней. Take a look at this: http://www.russianlessons.net/lessons/lesson11_main.php