Be sure to check the local calendar for any events that could affect your trip. While most local events and festivals just add to the enjoyment of a holiday, sometimes they can mean museums are closed, public beaches are crowded, restaurants are booked up, or transport is difficult.
If you're going somewhere you know is frequented by tourists from a certain country, it's worth checking their calendar too. Mexico in Spring may appeal, but bear it mind that's when thousands of horny American students descend for the infamous Spring break holiday!