When does the new day begin and end for Muslims, and does Friday begin after Fajr prayer or after Maghrib prayer?

Today, the term "day" is used legally to denote a 24-hour time unit, beginning from sunset to sunset, or from dawn to the next dawn, as in the Day of Arafah. The term "day" may also refer to the period opposite to night, beginning from the rising of dawn until sunset, as in the Prophet's (peace be upon him) saying: "Whoever fasts a day for the sake of Allah, Allah will distance his face from the Hellfire for seventy autumns." The legal definition of a day varies according to the rulings associated with it.