hello~ so I did a quick search and it looks like the "Night Mode" is an add-on patch to the original 2006 game. The game has a map movement gameplay where you choose where to move to interact with certain li's in their respective spots. The "Night Mode" apparently adds extra nighttime exploration which in turn lets you see additional scenes and reach the night endings(in addition to the ones in the main game for each character). Of course the patch includes extra spice.
The 2012 release called Butlers ~Meshimase Ojou-sama & Night Mode~ includes the original first press release and the patch in one purchasable bundle.
https://ameblo.jp/taiyo-326/entry-12588459757.html this user explained it and even reviewed the individual characters' routes.
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