It's always sunday when you load your save game on the Xbox 360. This can cause problems with merchant inventories and prevent you from resetting them.
Edit: to clarify, it's always sunday when you boot the game up from the dashboard. You can save your game on friday but when you start the game again it'll always be sunday, messing up Merchant inventories.
I just confirmed this bug post-patch, saved a game on monday, quit to the dashboard, started the game and loaded the save, its sunday again. I even started a new game to see if this was not a pre-patch save issue, but no, bug still happens in a new save.
How come your current day is not saved?
It's incredible the number of bug this game has that where part of a basic feature of the gamebryo engine, day management exists in gamebryo since Oblivion, how can they [censored] this up?