Shulchan Aruch: Chapter 256 - Six Shofar Blasts that Would Be Sounded on Friday

Shabbos 35b states:

Six shofar blasts are sounded on Friday: the first to alert the people in the fields to cease work, the second to alert the workers in the city and the storekeepers… he then waits the time it takes to roast a small fish or to attach a loaf to an oven [wall] and then sounds a tekiah blast, a series of teruos, and then a tekiah blast. Afterwards, one should rest.

Note the Kuntreis Acharon (note 1) to sec. 251, where the Alter Rebbe cites four interpretations of that teaching.

The term “holy communities” is commonly used to describe Jewish communities in general.

For this reason, many communities have instituted sounding a Shabbos siren or the like to alert the populace about the impending arrival of Shabbos.

I.e., when the Evening Service is recited at nightfall, as is customary in the Chabad community.

I.e., according to the more lenient opinion mentioned there. As mentioned in that source, there is a more stringent opinion that maintains that the restrictions should begin from minchah gedolah, which is a half hour after noon.

As explained in sec. 251 (note 7), the intent is shaos zemanios, “seasonal hours.” See the definition of the term there.

Note, however, that in his Siddur, Hilchos Krias Shema, the Alter Rebbe states that we measure “seasonal hours” from sunrise to sunset. Thus, these two and a half hours would be earlier, since the time would be measured from sunset, rather than from nightfall. In all such instances, the later ruling of the Siddur defines contemporary Lubavitch practice.

See subsection 2 and Kuntreis Acharon 1.

Sec. 251:3.

I.e., approximately a half hour before sunset.

Such commercial activity is obviously forbidden on Shabbos.

Devarim 1:13.

Sifri and Devarim Rabbah 1:10 explain that the letters אשם from the word ואשימם, translated as “I will place them,” can also be read as asham, meaning “guilt,” i.e., the guilt and responsibility lies with the leaders.

See the Alter Rebbe’s Seder Hachnasas Shabbos in his Siddur, where he elaborates on this theme.

I.e., even actual forbidden labors.

The recitation of Barchu signals the formal acceptance of Shabbos, even if recited before sunset(see 261:7). Therefore, people mistakenly thought that they were not bound to adhere to the laws of Shabbos as long as Barchu had not yet been recited – even if it was recited after nightfall.

I.e., they might desire to wait as a token of respect.

Cf. Koheles 2:14, as interpreted by Rashi. I.e., a wise man sees the consequences of his actions. So too, the congregation’s delaying the recitation of Barchu could lead to others desecrating the Shabbos. Hence, they should be careful not to delay its recitation.