The Shulhan Aruch (Orah Haim 550:3) writes that on the Shabbat immediately preceding a fast day, an announcement must be made in the synagogue informing the congregation of the impending fast. There is a paragraph in the Siddur with the text of the formal announcement ("Ahenu Bet Yisrael Shema’u…") that the Rabbi or the Hazan should recite for the congregation on the Shabbat before a fast day. If many members of the congregation do not understand the Hebrew text, the Rabbi must ensure to make the announcement in English.
The reason for this obligation is obvious. Many people, especially in the Diaspora, use only the secular calendar, and are thus unaware of the Hebrew date. Unless fast days are announced in the synagogue, many people might not be aware of the fast day and will thus eat and treat the occasion as an ordinary day.
Before which fast days in particular is the announcement required?
Since the purpose of the announcement is to alert people to an occasion of which they might otherwise be unaware, it does not apply to fast days which we have reason to assume that everybody anticipates. Thus, no announcement is required before Yom Kippur, since was can certainly assume that everybody is aware of the onset of Yom Kippur. (If a congregant does not know that Yom Kippur is coming that week, then he has much more serious problems to worry about than an announcement in the synagogue…) Likewise, no announcement is necessary on the Shabbat before Tisha Be’Ab, since several restrictions apply during the week of Tisha Be’Ab, such as shaving and wearing freshly-laundered clothes. The people are already observing practices in commemoration of the Temple’s destruction during the days leading up to Tisha Be’Ab, and thus no announcement is needed to inform them about Tisha Be’Ab. The same is true about Ta’anit Ester, which is observed on the day before Purim. People are certainly aware of the occasion of Purim, and they are therefore also aware of the fast on the day preceding Purim.
Other fasts, however, such as Shiba Assar Be’Tammuz and Asara Be’Tebet, must be announced on the preceding Shabbat, since we cannot assume that the congregants are aware of these occasions.
The Shulhan Aruch suggests a mnemonic device to help one remember this Halacha – a verse in the Book of Mishleh (16:26), "Achaf Alav Et Pihu" ("He closed his mouth upon him"). The word "Achaf" may be read as an acrostic representing the words "Ab," "Kippur" and "Purim." It thus alludes to the three fasts of Tisha Be’Ab, Yom Kippur and Ta’anit Ester, before which one may "close his mouth" and does not have to announce the occasion on the preceding Shabbat.
Summary: On the Shabbat before a fast day, an announcement must be made in the synagogue informing the people of the coming fast. This does not apply to the fasts of Yom Kippur, Tisha Be’Ab and Ta’anit Ester, since people are generally aware of these fasts and do not need an announcement.
