Well, on the third weekend of this month, there's a massive psytrance festival - accordingly, HHH is now on the fourth weekend.
Directions:
Drive in the direction of Mae Yao, towards Ban Ruammit.
When you get to the new superhighway connecting the airport and Phayao (i.e. the intersection just by Janpha Cottage), turn left and keep going until you get to the bridge over the river.
Don't cross the bridge; instead head under the bridge and park at the riverside. Start 3pm.