Shashti tithi is said to be a nanda tithi and hence auspicious tithi. It is ruled by Kartikeya and also krittikas. Venus is said to be the ruler of tithi. In Mahabharata Kartikeya had given boon to this thithi that krittikas will devour children and cause suffering to the native. There are 7 krittikas who are said to be wife of seven sages. As story goes, when the god Agni vomited Shiva’s semen in Ganges these Krittikas (except wife of Vashistha) descended and took bath in Ganges and became pregnant. But they could not bear the heat of semen and immediately released six fetus in Ganges which became one body and thus Kartikeya was born.
These krittikas became his foster mother and nurtured him. When Skanda became young, Krittikas asked him to meet their husbands, Skanda said “Sages will not accept you as wife, as you have taken semen of Shiva” Having heard this they became angry and said “Let us allow then to devour man, women and their children’ Skanda denied them this right to do mass murder and gave them another boon instead saying that they will be able harm their children until the age of 16 only. Shashti tithi becomes inauspicious when Shashti Tithi falls on Tuesday, Saturday and Sunday. If one fall ill in Shashti Tithi on Tuesday, Saturday of Sunday recovery will take time or even one dies if planetary period is bad. When Shashti tithi falls on a Sunday or Tuesday or Saturday whichever paksha it may be, Mrityuda yoga is formed.

But when Shashti Tithi falls on a Friday then Siddha Yoga is formed. On Thursday Thithi is not bad for the native but astrologically it depends on in what lagan one is born in and where is Mangal in the chart. If it has any relation with Sun and Shani native will face lot of problems. Sage Vashishtha says that Shashti gives children but also devours them when combination is bad and birth is in sandhya time before 1 hr or after 1 hr.

Krittikas were also cursed by Moon to become inauspicious because they opposed their husband and took the matter in the court of Daksha Prajapathi for neglecting them.

There are Skanda Shashti, Hal shashti and in West Bengal women do Ashvin Shashti vrat, in Jyesth month they do Jamai Shashti; many women do it in every month .. in north Hal Shashti vrat is done .. Hal shahti story can be read here ..