Legend has it that sometime long ago in ancient China, there is this stunningly beautiful girl named San Niang (三娘). She was hell-bent on getting married but for some reason or other, she offended the God of Marriages, Yue Lao (月老) who decided not to match-make her to anyone so she could not get married no matter what for the rest of her life. So angered, she swore bitter revenge and determined to cause trouble and break-up any new marriages she comes across. Thus the term San Niang Sha (三娘煞) was born. It was later said, 犯“三娘煞”的日子不是结婚吉日。每月“三娘煞”之日，即初三、初七、十三、十八、廿二、廿七，不宜结婚. Translated meaning, On days of San Niang Sha it is not auspicious for marriage. Every lunar month during the 3rd, 7th, 13th, 18th, 22nd and 27th, it is not suitable for marrying.
Actually there are known to be 2 versions of San Niang Sha days. One set is fake while the other set is real. The first set of dates listed above is supposedly the fake one but no one is willing to test the theory with their own happiness so they rather avoid getting married in these dates, just in case.
The real San Niang Sha days are said to be:
Any 3rd lunar day that also happens to be a Geng-Wu 庚午 day
Any 7th lunar day that also happens to be a Xin-Wei 辛未 day
Any 13th lunar day that also happens to be a Wu-Shen 戊申 day
Any 18th lunar day that also happens to be a Ji-You 己酉 day
Any 22nd lunar day that also happens to be a Bing-Wu 丙午 day
Any 27th lunar day that also happens to be a Ding-Wei 丁未 day
So effectively, there are only 6 days out of a whole year that can be truly be called San Niang Sha days so the chances of getting married in one is really relatively slim.
But having said all that, be it for a wedding day or any other major event, San Niang Sha days, along with a whole bunch of other inauspicious days born of Chinese folklore, are not seriously taken into consideration by Feng Shui or Date Selection practitioners when choosing an auspicious day.