Now you know how to calculate the weekday of any given date, but why is it called the doomsday algorithm?
The algorithm was developed by John Conway, best known for his game of life. While he was a brilliant mathematician he didn't come up with the algorithm from the thun air. It's actually based on an earlier algorithm developed by Levis Carroll (yes, that Levis Carroll. He too was a math professor. If you didn't know go read Alice in Wonderland again keeping this in mind. It is indeed a true mathematical wonderland). Conway just was the first one to come up with a method simple enough to be calculated in your head.
Looking at Conway's other works there is certain theme of duality in them. Life and doomsday... He also developed a game called angel and demon and was involved in some theorems about free will, while studying the determinism of the world (and math). There is no "free will" in mathematical world, everything is predetermined. The doomsday algorithm is a good example of this. While the dates and their relation to weekdays might seem quite chaotic there is certain order to them that makes things like his algorithm work.
While the doomsday algorithm can't be used to calculate the time the world will end it can be used to determine which weekday it will be. If you know the date.