In Microsoft Project, you can have many calendars. Right out of the box you will have at minimum the following calendars:
- 24 hours
- Night Shift
Whenever you start a new file from a blank project, the project will automatically have the Standard as default. You can build custom calendars by going to Project | Properties | Change Working Time.
Once you have all the calendars you will need for your project, you can start assigning them to the project, tasks, and/or resources. Therefore calendar assignment can be at the level of the whole project, for a particular task, or for a particular resource.
Remember that the calendar assigned to the Project will be the automatically used for all tasks within your schedule. If you enter the column "Task Calendar", you will see that all the tasks are set to "None". This means that all the tasks are being scheduled based on the Project calendar. It does not mean that no calendar has been assigned to them.
In your particular question, I interpreted it as you want to change what a particular task behaves on a particular day as either working or non-working. So I've built a calendar called "6/48 Calendar" meaning 6 days / 48 hours a week.
Once you have that, then you can use it as task calendars to change the working time for particular tasks. A note of CAUTION: Assign calendars to tasks sparingly. Only use them when necessary.
Below you can see how my example is using the Standard calendar for my project. Therefore tasks are not scheduled during the weekend:
Now I assign my new "6/48 Calendar" to Task 2 to see if my task calendar makes Saturday a working day: