The Songkran custom of building sand pagodas is called Chedi Sai.
During the festival, people may bring sand to temple grounds and shape it into small stupas or pagodas. The practice is associated with making merit and symbolically returning sand that may have been carried away from temple compounds on worshippers’ feet during the year.
Decorating the sand structures with flags, flowers, and other offerings turns the activity into both a religious observance and a community tradition. Customs vary between regions and individual temples, so not every Songkran celebration presents Chedi Sai in exactly the same form.
Chedi Sai should not be confused with Loi Krathong, the November festival in which decorated floats are released onto water. Both involve elaborate local customs, but they belong to different Thai festivals.