Whether you're planning a trip for business or pleasure, be sure to not only check your calendar as to when might be a good time to go, but also the calendar of your destination(s) you wish to visit.  Some points to keep in mind:

• Holidays - Official holidays are different from those of the U.S. in every country throughout Europe, and sometimes even in different parts of the same country.   Be sure to check when stores, markets and businesses will be closed, and avoid paying double or more for airfares, hotels and rental cars.  Keep in mind that in some countries, businesses have no option and must be closed on Sundays and/or official holidays.
• School and Summer Holidays - Many businesses close at some point during the summer to allow their employees to vacation with their families.  It's not unusual for small, family-run businesses to just hang a sign in the window that reads: "Closed. On vacation from August 1 - 22, see you when we get back."
• Winter and Other Holidays - Just as above, many businesses will close during the Christmas & New Year holidays.  Many of the vendors that I visit basically finish up business by December 15th and don't really get going again until the 2nd week in January.  Also, most schools and businesses will close down for a week during the spring, often around Easter (since Good Friday, Easter Sunday and Easter Monday are all official holidays in some countries).
• Seasonal and/or Cultural Events - A number of years ago at the end of April I was heading to the Netherlands on a buying trip and usually stay somewhere south of Amsterdam.  I was surprised that almost every single hotel and lodging was fully booked up.  When I asked why, it turned out that entire busloads of tourists were coming to look at the tulip festivals around Holland, especially Keukenhof - one of the world's largest flower gardens.  This can also happen during Christmas markets, religious activities or other events.
• Weekends / Sundays - In some countries around Europe, stores and businesses must be closed on Saturdays and/or Sundays.  This rule has become more relaxed in the past 20-30 years, but in Germany, for example, many stores traditionally close by noon on Saturdays and by law must be closed on Sundays.
• Call Ahead and Make Appointments - If you're hoping to visit antique dealers or other vendors, call ahead and make an appointment to be sure they will be available.  Plus, this gives them a "heads up" that you're coming and someone will be there to personally help you.
• Weather - Although no one can control the weather, keep in mind that many flea markets, even if scheduled, may not see many vendors show up if the forecast predicts heavy wind, rain or snow.  I therefore prefer to go to flea markets in the late spring or early fall, avoiding the busy and more expensive summer months.  Most markets shut down during the winter months.
• Jet-Lag - Plan at least a day or two when flying over to Europe to adjust to the new time zone.  Relax in a sidewalk cafe and do some people-watching.  You don't want to plan visits to either museums or businesses when you can barely keep your eyes open.
• Siestas - The siesta is common throughout the Mediterranean and Southern Europe. It is the traditional daytime sleep of Spain and popular in Italy.  In Dalmatia (coastal Croatia), the traditional afternoon nap is known as "fjaka" (from Italian "fiacca").  Be aware that businesses may close for several hours in the early afternoon but may be open later in the evening.
• Stay Flexible - Other unforeseen events, such as airline or train strikes, road construction, traffic jams, etc. can affect your itinerary, so build in some extra time in your schedule and remain flexible.