On the time for the Sabbath to begin, as has been mentioned, SDAs trace this back to the Creation story. As stated, all six days in that story seem to begin the day with sundown on what we today call the preceding day.
Gustave is correct when he stated that the phrase "evening and morning" is a Hebrew Idiom. However, I will suggest that he is not correct in his identification of that idiom as meaning that the day began with the morning light. SDA scholars of the Hebrew and Aramaic will suggest that the idiom involved in those passages in Genesis tell us that the period of time was 24-hour day, as we would call it today, as opposed to it being a day of some metaphorical length, such as a year.
Gustave's reference to Leviticus 23, takes me to the 5th verse which tells us that the Jewish religious holy days began and ended at evening. However, as I have said, SDA observance is not based upon Jewish law. Rather it is based upon the Creation story as found in Genesis. It should be noted that the SDA observance of the Sabbath is not even closely related to the Jewish observance of the Sabbath.