Nothing important gets lost. It just gets a new name, a new disguise, a new face, and hides in obscurity until it is needed again.
Most Sabbath keepers today follow the Jewish tradition. Jewish tradition says that day begins at evening time, at the same time the night starts. Latins believe that day starts at midnight. Untutored children thoughtlessly think thank day begins at daybreak.
Jonathan Sjordal's site links to a good article on the topic, A case for the 12 hour Sabbath, by Neil Gardner. Neil makes a comprehensive and detailed case from the scriptures. Perhaps most interesting are the traditions of the Jewish women. The Jewish women observed the Sabbath from sunrise to sunrise, sunrise on Saturday to sunrise on Sunday. By no means could the Rabbi's stop the women from doing this. Finally the Rabbi's convinced the Jewish women to start observing the Sabbath on Friday night. But the women continued to observe Sabbath on Saturday night too, until Sunday morning. Like childrens games and songs, this practice of the Jewish women was likely of great antiquity.
Neil Gardner's article is well written and informative, but also long and involved. Troy Miller just put up an article along the same lines, that is much more streamlined. Troy makes his case quickly and easily, like a pro. His article is here: When Does Scripture Say a Day Begins?
After reading Neil Gardner's article I thought: "He makes a good case, I should study this matter again." After reading Troy's article I thought: "This at least is the truth; day begins at day break, when you can no longer see at least three stars in the sky."
Like the Saturday sabbath, the daytime starting in the evening has a long history. Like the Saturday sabbath, it cannot be found in Scripture. If anyone disagrees, let him write in with his counter-arguments. I'll post the best ones.