The Jewish calendar reckons the days from evening to evening starting at 6:00 pm because of the Scripture:
"And there was evening and there was morning, one day" (Gen 1:5).
When it turns nighttime the day (a 24 hour period) ends and a new one begins. Therefore the day consists of two periods, the period of light (day) and the period of darkness (night).
Both Biblical and Talmudic literature make mention of a division of the night into three or four watches, the morning watch (Ex 14:24), the middle watch (Judges 7:19) and the beginning of the watches (Lam 2:19).
The Hour
The hour is divided into 1,080 bars (halakim). The Talmud states that a day has 24 hours, one hour has 24 "onot". The "Onah" has 24 "ittot". One "et" has 24 "rega’im."