This tab contains below details:

Working Calendar enables you to define working days and / or holidays for the project.

Working Days - Keep the boxes checked for all the weekdays that are working days. Remaining boxes should be unchecked.

Additional Holiday - Select the holiday date, enter a name for the event and then click 'Add Holiday' to add a new holiday to the project calendar. The working days and/or holidays defined under working calendar will be considered in cases wherein number of days are predefined by user and the task due date gets auto-calculated by the system such as in distribution groups.
To remove any holidays that are already defined, click on cross icon corresponding to the concerned record in the holiday list showing at the bottom.
All the holidays defined in working calendar will be listed based on their date in descending order.

Remember to click 'Save' to complete adding the project. Learn how to edit a project once created.