AV (Heb. אָב), post-Exilic name of the fifth month in the Jewish year. Occurring in Assyrian inscriptions, in Megillat Ta'anit, and all later branches of rabbinic literature but nowhere in the Bible, it is etymologically connected with abib (or aviv; "spring"), the pre-Exilic biblical name of the first month (Nisan); the verbal root vʾv denoting "fresh growth." The zodiacal sign of this month is Leo. In the present fixed Jewish calendar it invariably consists of 30 days, the first of Av never falling on Sunday, Tuesday, or Thursday. In the 20th century, Av in its earliest occurrence, extended from July eighth to August sixth and in its latest, from August seventh to September fifth. Traditional days in Av comprise: (a) First of Av, the anniversary of the death of Aaron according to Numbers 33:38 (in the masoretic text and all the ancient versions, except the Syriac), once observed as a fast (Meg. Ta'an., last chapter, Neubauer). (b) The Ninth of *Av, still observed as the strictest of the four fasts commemorating the destruction of the Temple, is the culmination of a period of nine days of semi-mourning. Connected with it are two special Sabbaths, the one preceding the fast called Shabbat Ḥazon (cf. Isa. ch. 1) and the one following the fast called Shabbat Naḥamu (cf. Isa. 40) after the respective Sabbath haftarah readings. (c) The Fifteenth of *Av, once a joyous popular festival, the main day of wood offering to the altar (Ta'an. 4:5, 8; Jos., Wars, 2:17). (d) The Eighteenth of Av, once observed as a fast commemorating the quenching of the Eternal Light in the Temple in the reign of King Ahaz (Meg. Ta'an., loc. cit.). Predominantly joyful in Temple times, with the fifth, seventh, tenth, and twentieth of Av as additional festive days of wood offering (Ta'an. 4:5, 8), this month's character became increasingly somber after the Romans' destruction of the Temple and as more and more national catastrophes occurred (or were held to have occurred) in it, with increasing restrictions on sundry expressions of joy, in keeping with the mishnaic ruling "When Av comes in, gladness is diminished" (ibid. 4:6).
Eisenstein, Dinim, 1; Guttmann, Mafte'aḥ, 1 (1906), 70f.; et, 1 (1947), 9–10.
[Ephraim Jehudah Wiesenberg]