Each day of the week has a stance of Buddha (sitting, standing, etc.), you find the weekday and time of your birth, then pray to the Buddha corresponding to you're day. There are 9 in all, Wednesday has 2 stances, one for day-time and one for night-time, and the 9th is a Buddha for when you don't know your birthday.
A good converter from Gregorian dates to day of the week can be found here: Fourmilab. To convert Thai year to Gregorian, subtract 543 years from the Thai year. Your calender is based on the birth of Jesus, ours is based on Buddha!
To learn more about Thai Buddhism click here. This is mainly a Thai blog, but with a large English subsection, just click the English flag top right.