Some countries require proof of certain vaccinations to enter the country.
You may learn that certain areas are experiencing conflict or are considered unsafe to visit and decide to alter your plans.
Check your calendar for any important dates that might conflict with your travel plans--you might need to push your trip back a few days so you don't miss your grandpa's 100th birthday.
Check the average weather at your destination for the time of year you are traveling.
Find out the high, low, and shoulder season for tourists at your destination by looking at their tourist bureau online.