If you are visiting Sukhothai Historical Park between 3rd December 2016 and 3rd January 2017, be sure to look out for the light show at Wat Mahathat.
This will take place every night between 6 p.m. and 9 p.m. Best to be there at sunset as there is usually a colourful sky behind the principal Buddha image.
From what I understand, it is not a show as such, but rather an opportunity for people to go and take pictures of Wat Mahthat lit up at night.
There is no admission during this period. In fact, until 31 January 2017, all historical parks in Thailand are free.