Complex sentence: Loy Kratong coincides with the Lanna (northern Thai) festival known as "Yi Peng", which, due to a difference between the old Lanna calendar and the Thai calendar, is held on a full moon of the 2nd month of the Lanna calendar.
Split into simpler sentences: Loy Kratong coincides with the Lanna (northern Thai) festival known as "Yi Peng". Due to a difference between the old Lanna calendar and the Thai calendar, "Yi Peng" is held on a full moon of the 2nd month of the Lanna calendar.