When does Shabbat end and how do you know?

Shabbat ends when three medium-sized stars become visible in the night sky — a halachic marker called 'tzeit hakochavim.' In practice, this is calculated astronomically and published in calendars. The time varies each week and by location. Typically it is about 25 hours after Shabbat began, roughly 40 to 72 minutes after sunset depending on the custom followed (Geonim, Rabbeinu Tam) and on latitude. Until that moment every Shabbat prohibition remains fully in force, so one does not turn on lights, cook, or handle money. After nightfall the Havdalah ceremony is performed over wine, spices, and a flame, formally concluding Shabbat and marking the transition back to the weekday.