Question

how a new day begins

Answer 1

Different cultures have used midnight, dawn, sunset or even mid-day.

Midnight is the most common, because most people are asleep. The Romans used it and we inherited it.

But religious Jews use sunset. Their Sabbath begins on what I’d rate as Friday sunset and ends on Saturday sunset.

Incidentally, most Christians have muddled traditions. Early Christians joined in the pagan day of rest on Sunday. Later Jewish Sabbath rules were applied to this day, without good justification.