In Outlook’s Month view, the current day is a bit washed out. As you can see below, the default blue is a tad lighter than other highlighted areas. It isn’t impossible to find, but it does seem to fade into the background. (It’s even more obscure in Outlook 2003.) The color is in keeping with the theme, but if you want the current day to pop out a bit, try changing the default color.

In Outlook 2003 and 2007, do the following to change this property:

  1. From the Tools menu, choose Options and click the Preferences tab (if necessary)
  2. On the Preferences tab, click Calendar Options in the Calendar section.
  3. In the Calendar Options section, choose a new color from the Default Color dropdown.
  4. Click OK.

If you're using Outlook 2010, do the following:

  1. Click the File menu and then choose Options.
  2. Click Calendar in the left pane.
  3. In the Display Options section, choose a new color from the Default Calendar Color dropdown.
  4. Click OK.

This property changes the Calendar color, not the selected day, so it's a big change. You'll want to choose a color that contrasts with the current day's border, as shown above. In this case, the orange border is easy to see next to the green--at least, I think it is. Personal preference strongly figures into this particular choice.
Keep in mind that all of the calendars, not just Month view, will be updated. In addition, the system you're using might matter. I'm running Outlook 2007 on a laptop and the colors are more subtle than in Outlook 2010, which I run on a desktop system. The colors might actually be a bit different, but it might be the differences between the two systems as well.
This tip won't set your world on fire or anything. It's just one of those simple things that you can control, so you should if if makes your day a bit easier!