Delete the duplicates in your Outlook: NoMoreDupes for Outlook
Your calendar in Outlook

11. The timing of my appointments is wrong by one hour. Some whole day events are spreading over two days.

Check your time zone settings both in Outlook (Tools | Options | CalendarOptions) and your PC (Start | Settings | Control Panel | Date and Time) to make sure they are correct.
To fix the existing appointments, set your clock back so the appointments are at the correct times. Export your appointments to a comma separated values (CSV) file. Now, create a new calendar folder and move the appointments to it (as a backup).
Finally, set your clock back to the current, correct time and import the CSV back into Outlook.

12. Outlook 2000 does not display holidays
Microsoft provides an update for holidays:

13. I want full-sized boxes in the calendar for Saturday and Sunday instead of a split box.

Click on View > Current View > Customize Current View > Other Settings
Clear the box for "Compress weekend days".

14. Is there a way of adding tasks to the outlook calendar?

Wisco provides an Outlook addin to display your tasks in your calendar automatically.
Furthermore your tasks will be kept in sync with your calendar. Even the state and the notes of the task are included in your calendar:

15. I want the name of the day/month, not the number, to display and print on my calendar.
You need to change the long date on the computer to use names.
Click on Start > Settings > Control Panel > Regional Settings and change the date settings.

16. When I send an appointment from Outlook 2003 to 2000, it only appears as a text message, and there is no option to add it to the calendar.

Omit the reminder. Outlook 2000 has an iCalendar incompatiblity

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