He was fasting then he traveled to another country where they started fasting later; should he fast thirty-one days?

Differences in moon sighting between countries and its effect on those who travel from one to another

Can You Treat a Toothache While Fasting?

It is not permissible to differ from the people of the city with regard to fasting and Eid

They found out during the day that Ramadan had begun, so they fasted

Is it necessary for every single Muslim to see the new moon of Ramadan?

Can Muslims living in Western countries form committees to sight the moon?

We should rely on sighting of the moon, not on calculations

Is there a certain length of time that the new moon has to be in the sky?

It is permissible to use telescopes etc., but not calculations, to sight the new moon