Kingdom Hearts 358/2 interview via Famitsu.

"Interviewer: First of all, please tell us the meaning of the title 358/2 Days.

Nomura: The title was done on purpose as a code, you can only understand it after beating the game. The word Day, is attached and the progress of the game is to show how the Organization came to be, as well as the daily life of the Organization XIII. They are always faced with the duty from a place, and then have to return. The game will focus around Roxas, who is a member of the Organization. It will be an adventure game, repetition is also one of the concepts of the game."

Full Source: KH-vids.net via Gonintendo