Setting up what is considered a workday vs non-work day helps to ensure proper start and end dates for your schedule items.
For example, lets say you have a schedule item that is suppose to take 3 work days and your work days are set to Monday - Friday. If the item was to start on a Friday, it would end on Tuesday of the following week. Friday (Day 1), Monday (Day 2), Tuesday (Day 3).
If you change your work schedule to Monday - Saturday, this example item would be set to complete on Monday. Friday (Day 1), Saturday (Day 2), Monday (Day 3).
Here is how you set your workdays in your account: