Timesheet Holidays
Since version 2.6 it is possible to define variable or fixed holidays in Administration - Add Ons - Timesheet Holidays.
Such holidays then appear as days off in Time Sheet gadget and report.
Holidays Calendar
You may use Holidays calendar to specify holidays manually.
Import Holidays
Or you may import .ics with corporate or state holidays. See calendarlabs.com for example
See also Issue#297