To add to Paul's excellent answer, you would need to handle this based on your scheduling needs.
1. To make every Saturday a working day for every project, you would need to check out the enterprise Standard calendar and set Saturday as a working day according to your organization's normal schedule (such as 8:00 AM - 12:00 PM and 1:00 PM to 5:00 PM). Once you save and close the enterprise Standard calendar, Microsoft Project would automatically schedule very task in every schedule using the 6-day work week schedule. In addition to this, however, you would need to instruct your PMs to change a Project Options setting in response in each of their projects. On the Schedule page of the Project Options dialog, tell your PMs to change the Hours Per Week value to 48h to correspond with the changes to the enterprise Standard calendar. By changing this setting, if the PM enters a Duration value of 1 week, Microsoft Project will automatically set the Duration to match the 6-day work week schedule.
2. If you only need to give you PMs the option to schedule certain tasks on a 6-day work week schedule, then follow Paul's excellent advice. You will need to create a new enterprise calendar named something like 6-Day Work Week, leave Monday through Friday as working days, and then set Saturday as a work day as well. Your PMs would then be able to use this custom calendar as a Task calendar to override the schedule of individual tasks to schedule them on a 6-day work week schedule.
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