The words every administrator hates to hear: "it was always like that at my old job" came with a request from a user that when she opens Outlook, she wants several calendars open at the same time, automatically.
Yes, it is possible to have Outlook open multiple windows when it loads.
Open the Outlook windows you need and arrange them to your liking then always close Outlook using File, Exit (from the "default" Outlook window). All of the windows will close at once and Outlook restores them the next time you use it.
Always close the "default" Outlook window using File, Exit (to close all windows at once) or when using the X if you want all of the windows to load next time. (The default window is the window the Outlook shortcut opened, not one of the windows you opened.) If you close the windows you opened first, Outlook won't reopen them.
Always Open Selected Calendars
A popular request is the ability for Outlook to remember what calendars were selected when you last used Outlook and to reselect them at startup.
If you use Outlook 2003 want to open the Calendar module with the same calendars checked each time you'll need to use a Windows macro or scripting utility such as AutoIt to automatically enter the required keystrokes the open these other calendars. Utilities should not be needed for Outlook 2007 and up; selected calendars should be selected until you deselect them.
Outlook 2007 and up should remember the selected Calendars, although it seems to only work 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 reselects them for Outlook 2003 and 2007.