The Project Calendar is used to schedule tasks by default until a resource
is assigned. When you add the resource to the task, the resource's calendar
takes over the scheduling control. Think about it for a moment - you can't
do the work on a task unless the person doing it is at work - so the
schedule of the task should normally follow the resource's fixed work
schedule. If you edit the Standard calendar to include holidays, or if you
copy the standard calendar to a fresh calendar and add the holidays there,
you can then make the resulting calendar the Project Calendar in the
Project/Project Information menu. You can also make that the base calendar
for the resources, thus causing the individual resource calendars to pick up
all that holiday infomation as well. There's no need to use Task Calendars
at all for most of the tasks - the reason they exist is to allow for those
odd-ball exception conditions where tasks must be driven by something other
than the Project Calendar or the assigned Resource Calendars, and in many -
if not most - projects they are unecessary.