Calendars are necessary to define working programs for resources and tasks.
They can be created and modified from Calendar step in Project Guide or Settings menu, Calendar.
Company calendar is the calendar used by most of the resources. It is the default calendar for every new human resource as the Project calendar is the default calendar for project's activities.
Although the predefined calendars can be modified it is not adviseable to do it.
If a resource need a special calendar (on a regular basis, not just for some special days) select in Calendar editor a calendar with the closest working program to the one you need, then press New button to create a copy of it (e.g. if Standard calendar was selected, the newly created calendar name is Copy of Standard). Give it a suggestive name, as the combo box is editable. To change the working program, first step is to select working days by marking them in checkboxes. It is possible to select more working days with the same program (using Ctrl or Shift) and change them at once.
For tasks is necessary to make new calendars even for the slightest difference in the working time comparing to other existing calendars, as the tasks doesn't have the possibility to define special days (this situation is far less probable to occur than for resources).
Important: changing a calendar already in use will influence the schedule accordingly!
To define a special program for certain days click on Set holidays, days off and other special days. Select the date in the calendar and modify its program or mark it as Nonworking day. To quickly get back to the normal program, as defined before at Set working days, select Week defined program. Custom program will associate to the selected day(s) a predefined program that can be changed from the Settings menu, Options, Schedule tab.
Delete the unnecessary calendars by clicking on Delete button. If you attempt to delete a calendar already in use, you'll be warned about that and invited to replace it with an existing one (select from drop down list and press Replace).