Siman 256

When Israel was on its foundation, they would sound six blasts on the eve of Shabbat after midday. And not specifically a tekiah, for tekiah includes everything, and thus they would sound: tekiah teruah tekiah, tekiah teruah tekiah. (Rashi 35b, starting with "Six") And these blasts were signals for the preparation of the Shabbat, as will be explained. And according to the opinion of the Rambam at the end of chapter 5, also on the conclusion of Shabbat they would sound one tekiah after the emergence of the stars, to inform that labor is permitted, and the Ramban and the Rashba disagreed with him, for the sounding is only at the onset of Shabbat and not at its conclusion.

What is the order of the blasts: The Rambam wrote there, that he would blow in a high place in order to make it heard throughout the entire city and behind the city. The first blast was after mincha gedola, and at the first blast the people in the fields desisted from plowing, from hoeing, and from performing any labor in the fields; and those nearby are not permitted to enter the city until those far away hear and they all enter as one, so as not to come under suspicion. The second blast they would blow at the time of mincha ketana, which is two and a half hours (Magen Avraham) before nightfall; they lock the shops (Gemara 35b) and the craftsmen in the city desist from performing their labor. And afterwards, after some interval, they blow a third blast, and then they removed the pots from the stoves that were prepared for the evening meal, and they insulated in hatmana the pots that were prepared for the next day, and they lit the candles. And afterwards they wait a small amount of time, enough to roast a small fish, and they blow three blasts, which are a tekiah, a teruah, and a tekiah, and they accept the Shabbat. And the blower hides the shofar in the place where he blew and shall not carry it to his house, for they have already accepted Shabbat upon themselves (Gemara 35b); and I do not know why the Rambam omitted this. And when Yom Tov falls on the eve of Shabbat—nevertheless they would blow, for the sanctity of Shabbat is more stringent. And it seems to me that the first two blasts were not then, for on Yom Tov one does not perform labor and does not engage in trade. And on the conclusion of Shabbat they would not blow even if Yom Tov falls on Sunday, for there is no blowing except in a place where a more stringent sanctity enters.

Our teacher, the Rema, wrote that it is the custom in holy communities that an hour before Shabbat, the shliach tzibbur announces to prepare oneself for Shabbat, and this is in place of the blasts in their days, and so it is proper to practice. Thus far his words. And now even this is impossible, but we send a specific man to drive [people] from the market and from the shops, for there is a great stumbling block in this, as most often toward evening the work of the Satan causes them to come to buy, and this requires much strengthening, and it is clear that the holy Shabbat repays those who are careful with it. And one should hasten the prayers of Kabbalat Shabbat and Maariv while it is still day, and one should not wait for a great man who has not yet come to the beit knesset, and it is a great merit. (Magen Avraham) And in our many sins, they now delay very much to pray, and they consider it a mitzvah to pray Maariv in its time. And woe to this mitzvah, for it is tested and proven that through this many of the city's people come to desecration of Shabbat, and we have already elaborated on this in siman 242, and happy is the man who will do this to exert effort so that they pray Maariv while it is still day, and there is no end to his reward and Shabbat will advocate for him.