How the date is fixed
Ahoi Ashtami falls on the eighth day of the dark half of the lunar month Ashwina (Krishna Ashtami). The date is the day on which that tithi is running at pradosh, the twilight just after sunset. In the North Indian (purnimanta) calendar the same fortnight is counted in Kartika.
About
Ahoi Ashtami falls on the eighth tithi of the dark half of Kartika as North India counts it (purnimanta), which is Ashwina Krishna Ashtami in amanta reckoning, in October or November. Karwa Chauth comes four tithis earlier and the Diwali new moon seven tithis later, so the gap in calendar days changes from year to year. The fast is broken in the evening at the first sighting of stars, so the day whose dusk carries Ashtami is chosen. An image of Ahoi Mata is drawn or hung on the wall and the story told before the stars appear. It is kept in the same belt as Karwa Chauth and often by the same households.
How it is observed
A day-long fast by mothers, the Ahoi story at dusk, and water offered to the stars before eating.