Calendar:Adding a holiday calendar

  * Find a holiday calendar.  Many are here.
  * Copy the link location.
  * Open Thunderbird 2 (with lightning installed).
  * Switch to the calendar view.
  * Choose 'File' / 'New' / 'Calendar...'
  * Choose 'On the network'
  * Click 'next'
  * Choose 'ics' as the format, unless you know the format is 'CalDAV'
  * Paste the location you copied above.
  * Click 'next'
  * Choose a name for the holiday calendar.
  * Click 'next'
  * Click 'finish'

This will now load the holiday calendar, most likely as read-only.