What's the day today (or other dates)?
PHP - 101 97 103 125 characters

Sakamoto Algorithm
0 = Sunday

Code
&lt;?php fscanf(STDIN,&quot;%4d%2d%2d&quot;,$y,$m,$d);@$a=a032503514624;$y-=$m&lt;3;$z=$y+1;echo($y+$y/4%$z-$y/100%$z+$y/400%$z+$a[$m]+$d)%7;

Note
Unfortunately, due to PHP's dynamic, weak typing, the Sakamoto algorithm doesn't function properly without explicitly flooring each division operation.