In general, September usually the wettest month of the year in most parts of Thailand whilst temperatures continue to drop slightly in some parts.
The festival takes place on the full moon closest to the autumn equinox. The Mid-Autumn Festival is held on the 15th day of the eighth month in the Chinese calendar, which is in September or early October in the Gregorian calendar. It is celebrated with Chinese communities across Thailand. Shops will sell special moon cakes to mark the occasion.
September is just simply the continuation of conditions in August. Most of Thailand dominated by monsoon rains though more or less. In cities, street flooding is expected sometimes. Trekking and visiting to the remote mountainous areas become difficult as the condition is so wet. Landslide can be another problem in deforested mountainous areas.
A classic style holiday to Thailand during this time is more appropriate than a big ambitious adventure itinerary. Most places in the central Thailand are accessible and enjoyable, the same as in lower north and upper eastern Thailand. Avoid the rains in certain times of the day you will find it enjoyable traveling in Thailand during this time.
An umbrella or light poncho is needed when you travel around if you do not want to get wet because of sudden showers.
This time sees places in the country less touristy so, this is an advantage you can take and good promotion is another thing to think about.
If you want to find sun, sea and sand then Pattaya, Koh Samet and beaches and island on the southeast coast are your best places to visit Thailand in September.
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