How were people informed of the time when Shabbat began in an era without clocks or modern communication?
The Mishna on today’s daf discusses the role of the tekia, or shofar blast, that served that purpose.
According to the Mishna in Sukka (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 was approaching and that they should put aside their work in the fields or in their businesses in order to prepare for. Later in the afternoon there were three more shofar blasts that warned that 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 Rabbeinu Ḥananel suggest 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 Mishna on today’s daf, which teaches that when havdala was made the shofar was not sounded. The Ohr Same’aḥ suggests that the Rambam is based on the Gemara in Shabbat (daf 114b) which is discussing the practice in Babylonia, while our Mishna refers to the practice in the Temple.
The description of the shofar blasts in the Mishna 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.