Since the project calendar defaults to five working days. You only need to customize the weekend working hours.

So, to have a 7 day work week simply go to the Tools -> Calendar Editor menu. Once in the Calendar Editor window, click on the Saturday where you wish for your 7 day work week calendar to start. Under Set work hours (right pane of the Calendar window):

1.    Set the number of work hours (0-24),

2.    Set the frequency (One time, daily, weekly, every two weeks, monthly by date, yearly), and

3.    Set the duration of the recurrence (the number of frequency units (i.e., days, weeks, ...) for which this setting should apply) .

4.    Click on the Apply button.

Your Saturdays are now working days. Repeat the same procedure for the Sunday.

You will notice that now Saturdays and Sundays are NOT highlighted, indicating that they are working days same as the rest of the week.