According to legend, San Niang 三娘 was stunningly beautiful, in Chinese we called her beauty as 倾国倾城 which means her beauty causes the downfall of cities and countries. She wanted to marry her Mr. Right but no one know what was the reason that she did offended the God of Marriage and this cause her unable to get marriage in her entire life. San Niang wanted revenge and she vowed to create havoc at every marriage.
Therfore, in ancient Chinese, couples try to avoid San Niang Sha days which falls on the 3rd, 7th, 13th, 18th, 22nd and 27th day of every lunar month.
However there are two versions of dates. The above are considered as the fake one. The true one are
3rd day fall on Geng-Wu 庚午
7th day fall on Xin-Mei 辛未,
13th day fall on Wu-Shen 戊申,
18th day fall on Xi-You 己酉 ,
22nd day fall on Bing-Wu 丙午 and
27th day fall on Ting-Mei 丁未.
Which only means six days.
San Niang Sha is a type of old folk story method use for date selection that was recorded in traditional Almanac. There is lack of theoretically basis and it is in conflict with conventional method of date selection. Therefore, many professional geomancer do not refer to San Niang Sha when they are working on FengShui as well as in BaZi Four Pillars of Destiny.