Which day is designated as Ashura, and what is the recommended practice for fasting it?

Primary text

Ashura is the tenth day of Muharram, which is the view held by Sa'id ibn al-Musayyab and Al-Hasan, based on the narration from Ibn Abbas that the Messenger of Allah commanded fasting the tenth day of Muharram. It is recommended to fast both the ninth and the tenth days to avoid resembling the Jews, as stated by 'Ata'. Ahmad explicitly stated this recommendation, as did Ishaq. If the beginning of the month is ambiguous, one should fast three days to ensure attaining the fasts of the ninth and tenth.