A popular request is the ability for Outlook to remember what calendars were selected when you last used Outlook and to reselect them at start-up.
The good news: Outlook 2007 and newer versions should remember the selected Calendars.
The bad news: it only remembers when you “start in” the Inbox or non-calendar folder.
If the calendar module opens at start, only the default calendar is selected. This includes using the Start in [folder] option or closing Outlook with a second window open to the calendar module. Using Open in new window command will also display just one calendar.
If you want to open to the calendar module and remember which calendars were selected, Public Calendar Choice (freeware) remembers selected calendars and re-selects them for Outlook 2003 and 2007.
You can also use a VBA macro to select the calendars. See Select multiple calendars in Outlook for details.