Microsoft Outlook in each and every version is missing some obvious, yet very important functionalities and Microsoft’s dumb-asses are reluctant into hearing users voice and adding them. One of such very-missed features is greying-out passed days or highlighting current day.
Without them each and every day in Outlook Calendar looks just the same and ultra-fast distinction between today’s events and passed events is simply impossible.
Fortunately, some workarounds do exists. They may not be enough for all of us, but at least they allow you to learn a little bit about conditional formatting that you can use in Outlook Calendar or Outlook itself (for i.e. colouring events and e-mails etc.).