How were people informed of the time when Shabbat began in an era without clocks or modern communication?
The Mishnah on today’s daf (=page) discusses the role of the tekiah, or shofar blast, that served that purpose.
According to the Mishnah in Sukkah (daf 53b), during Temple times the tradition was to blow the shofar three times on Friday in the early afternoon in order to indicate to the people that Shabbat was approaching and that they should put aside their work in the fields or in their businesses in order to prepare for Shabbat. Later in the afternoon there were three more shofar blasts that warned that Shabbat was imminent; with the final sound of the shofar, all work was forbidden. Similar announcements were made in Jewish communities throughout the generations. Rashi and Rabbenu Hananelsuggest that a similar series of shofar blasts were sounded on the eve of holidays, as well, even though there is no clear source for this in the Gemara
The Rambam writes that there were shofar blasts sounded at the close of Shabbat, as well, whose purpose was to permit the people to return to work. This appears to contradict the Mishnah on today’s daf, which teaches that when havdalahwas made the shofar was not sounded. The Ohr Same’ah suggests that the Rambam is based on the Gemara in Shabbat(daf 114b) which is discussing the practice in Babylonia, while our Mishnah refers to the practice in the Temple.
The description of the shofar blasts in the Mishnah is supported by Josephus who records that there was a tower in the walls of the Temple Mount where the trumpeter stood and sounded the shofar on Friday afternoon in order to announce that the laborers should stop their work. In fact, archaeological excavations in the area of the Southern Wall of the Temple have uncovered the remains of a large stone fragment, which has inscribed on it “…to the trumpeting house.” This rock was discovered in the south-west corner of the Temple Mount which faced the business district, as well as the fields on the slopes of the Jerusalem hills.