This time of year (January, February) is when the sugar cane is harvested, in Kanchanaburi province, the streets are filled with convoys of trucks carrying the harvested sugar to the factory for processing. At this time of year you'll also see sugar cane sap vendors on the street with a machine to squeeze out the sweet juices and freshly cut canes. The cane sugar for drinking is much more juicy than the cane sugar for making sugar, it is grown specially for this drink.
Here you see the press, the cane is pushed into the press folded over and pressed through again and again until all the juice is squeezed out, you can see the juice running out into the yellow jug at the back. Pour it over ice and that is it! Fresh clean sweet juice.
You do need the sugar cane press to make this drink, so sadly it's one drink you can't make at home, but if you're in Thailand and see vendors with sugar canes, and a press, then why not try it?