One day refers to ONE day. Monday is one day and so is Tuesday, Wednesday never really fit in with the other days but it is near enough one day, it has different hobbies and points of view, but lets not get into that. The other days are also one day, if they are separate.
One day that is historic is the day Jesus was born. Yes you guessed it... it was Christmas Day. This day is historic because Jesus was a Jew. Oh and because his mum was a virgin, crazy story really. Jesus went on and got a "crew" of adults to walk around with him whilst he made money on the side buying cheap stock and flogging it down the market. But from his words of wisdom, a new religion was born, Christianity. See amazing what can happen on one fateful day, all of that stuff came from one day!
Another one day event is Halloween. This is a day 1 to 11 year olds ,and also some 19 year old kids with no friends, walk around pretending to be scary to rob people of their sweets!! An outrage it is. But from this event a horde of fat children were born from eating too much, this again demonstrates the power of one day.
There are some also less important one day events such as the 11th of January and 25th of June. These are not so important because some wierd old people didn't make up special events on these days. They've always felt left out since.
Umm what else happens in one day... oh! Hitler was born on one day somewhere and look what he did! He got a messed up haircut and a moustasche, oh and killed some people some day. (I don't think Hitler liked the Jews that much).