Evening to Evening
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).
Daytime to Nighttime
The transition from daytime to nighttime, from light to darkness, and vice versa, is very gradual. Daytime ends just before sunset, and continues until shortly after sunset. Daytime begins just before sunrise and continues until shortly after sunrise. The two periods of transition cannot be defined very accurately, and they are called "erev" and "boker" (evening and morning). There is also a word in the Hebrew "neshef" (dawn and twilight).
Note: The rabbis fixed the duration from daytime to nighttime to be thirteen minutes, thirty seconds before night.