Your default calendar is the calendar folder in whatever folder set you're
using as your delivery location.  It's the one that shows in the Navigation
Pane under the root of your Personal Folder folder set or your mailbox,
depending on account type.  It's the calendar into which the ites will be
stored if you click "Accept" for an invitation or "Save and Close" for items
you create yourself.  Outlook Today will show you whatever events are
scheduled in that folder for whatever period you've configured Outlook Today
to show (one to seven days, although you can go longer than that if you
modify the registry directly).  If you'e used "Customize Outlook Today" and
have chosen a time span for how many calendar days will be shown, if your
events fall outside of that time span, then they won't show.  If they're in
a folder other than the default calendar, they won't show, either.