Thanks to Karen Turner from Turner Efficiency for this awesome tip:

Do you:

• do business in another country?
• have family living elsewhere?
• travel?

Add that country’s holidays to your calendar.

Being culturally cognizant can help you successfully close a business deal, remember to wish someone well, and/or travel at low seasons for better fares.

You can even add non-statutory holidays (such as International Women’s Day) or sports calendars.

Outlook 2010 & 2013
File>Options>Calendar>Add Holidays>Choose

Outlook 2003 & 2007
Tools>Options>Calendar>Add Holidays>Choose

Gmail
Google Calendar Sign in>Other Calendars>Add>Browse Interesting Calendars>Subscribe

iCal
iCal>Calendar>Find Subscriptions>Choose
OR
Calendar>Preferences>General>Show Holidays