=============== wikipedia has an interesting historical article on how that came about.
from:
Sabbath in seventh-day churches - Wikipedia.
In 1851, Adventists taught that the sabbath begins at 6PM Friday, and not at sunset, nor midnight, nor sunrise:
[52]
It is clear, therefore, from Scripture testimony that every day commences at 6 o’clock, and not at sunset, nor at midnight, as many contend, nor yet at sunrise as some others believe. Therefore the Sabbath commences at 6 P. M. on what is called Friday. Every hour and minute of it is sanctified time, “holy to the Lord, and holy to those who keep it. (ARSH April 21, 1851, p71.7)
The Adventists held a conference at Battle Creek, Mich., Nov. 16, 1855. At this conference, they voted to accept J.N. Andrews's decision that the Sabbath begins at sunset:
A division among them was arising over this question. So Elder J. N. Andrews, the best scholar they then had, was requested to study the subject and present his conclusion to the conference held at Battle Creek, Mich., Nov. 16, 1855. This he did, and decided that sunset was the Scriptural time to begin the Sabbath. The conference voted to accept his view.... “Then, four days after Andrews and the conference had settled it, Mrs. White had a vision in which an angel told her that sunset was the right time!!! ... In that vision she complained to the angel and asked for an explanation. She says: ‘I inquired why it had been thus, that at this late day we must change the time of commencing the Sabbath. Said the angel, “Ye shall understand, but not yet, not yet.”’ (‘Test.,’ Vol. I., p. 116).