How to change a standard calender

[LG]

Hello,
Some one please help me. I have a project with a standard calendar. I edited
to change the calendar through the system account as the weekends changed.
i.e to include friday as non working day. When I try to edit the project
plan i.e. create new task using microsoft project 2007, and if i include
fridays Iam not prompted with a message saying friday is a non working day,
where as for a new project using the same calendar it prompts the message.
please help