*sigh*
I was kinda hoping you'd just go with it so I wouldn't have to type up the entire explanation, hah. But OK. That's what I get for being lazy:
When Halo 3 was released, you had to get all the achievements to unlock the katana. That entailed getting 49 achievements which equaled 1000 GamerScore (GS).
In 2008 the Legendary Mappack was released. This mappack came with new achievements. These achievements, when released, caused a glitch in the game. Suddenly the katanas were popping up everywhere. Some people had 300 GS, some had 1200 GS and didn't have it.
The unfortunate truth is that now it unlocks randomly when you get an achievement. However, this same glitch also relocks the katana for people when they do unlock it, quite often (yes, some people get away with it never relocking itself).
The only way to ensure it never relocks itself is by obtaining the original 49 Halo 3 achievements, which is how you actually earn it (after all, anyone getting it without them just got it from a glitch and never actually earned it).
So I said 'get the original' 49 because... it was easier.