All of G-d's work was finished in six days. (Genesis 2:1) This is why we must finish our work in six days, as a sign that our work is finished we rest. If we refuse to rest, our work has not ended. You end work literally when you rest, it is unlawful to do any labor during the seventh day, your work has ended. Evening is the end of a day, morning is the beginning. Man must work while it is day, the night cometh no man can work. G-d call the light day, he called the darkness night. We know speaking during Sabbath is not unlawful, YHWH spoke the world into existence. It is lawful to do good on the Sabbath; YHWH did only good each day. As you stated to do good is not labor or work. Sabbath has no evening and morning, it is eternal, holy, a spiritual day. Sabbath means rest. When a lawyer in a courtroom says he rest his case, the entire courtroom know he has finished speaking his argument.