Why is the Karva Chauth fast broken at moonrise?

After the full moon, the moon rises later each evening. By the fourth night it usually comes up a couple of hours after sunset, so a fast that ends at moonrise is one that runs well into the night. It also comes up at different times in different cities, which is why your cousin in another city may eat before you do.

The date itself is set by the sky: the festival belongs to the day when Chaturthi of the dark fortnight is in force at moonrise. In north India this fortnight is called Kartika, and in the west and south, Ashvin. It's the same fortnight under two names.

The tale usually told at the evening gathering is about a young bride, often named Veeravati, fasting at her parents' home. She grows faint. Her brothers can't bear it, so one climbs a tree with a lamp and they tell her the moon has risen. She eats, and her husband falls dead. The goddess, Parvati in some tellings and Indrani in others, tells her that if she keeps the fast faithfully he will be restored.

Notice what the story guards: the real moon, not a convenient substitute. Other families tell of Karva, a devoted wife who confronted Yama over a crocodile, or of Savitri. These are carried by word of mouth and vrat booklets, and the details change from house to house.

The fast was traditionally kept by married women in parts of north India, particularly Punjab and parts of Uttar Pradesh. Its spread across the country is recent and well documented. Sabrina Ciolfi of the University of Milan describes Dilwale Dulhania Le Jayenge (1995) as a real turning point, after which unmarried women began fasting for boyfriends, couples began treating the evening a little like Valentine's Day, and advertisers took notice.