What is "samjinnal"

Samjinnal is one of sesi pungsok or Korean traditional customs by season, which falls on the third day of the third month in the Korean lunar calendar. It was called samjil in old Korean language and referred to as sangsa , wonsa , sungsam , sangje or dapcheongjeol in hanja. Samjinnal implies the overlapping of Sam (three). According to Choi Namseon, samjil was derived from the consonants of Samil, and Sangsa is defined as the first snake day of the 3rd lunar month.