Fortunately, others have gone to the trouble of writing files full of useful stuff--and you can load this stuff into Mathematica for your very own use. Some people produce useful stuff and you can buy it, which is nice if you find it valuable--and you can write stuff and gain value by selling it, which might be even more nice.
Mathematica comes with a group of Standard Packages, that you can load in to do special tasks. The Package documentation can be found with the Helper Palette, available here. For example, take a look at the specialized package Calendar:
| Created by Wolfram Mathematica 6.0 (28 August 2007) |  |