NISAN (Heb. נִיסָן), the post-Exilic name of the first month of the Jewish year. Its pentateuchal name is ḥodesh ha-aviv (lit. "month of spring," Ex. 13:4 and parallels) and it is also referred to as the month of the ripening ears of barley (ibid. 9:31). The post-Exilic name, occurring in the biblical and apocryphal records (Esth. 3:7, Neh. 2:1; I Esd. 5:6, Add. Esth. 1:1) and frequently in Josephus and rabbinic literature (e.g., *Megillat Ta'anit), is linked with the Babylonian first month, Nisannu (derived from nesa, Heb., nasa "to start"). The Mishnah calls the first of Nisan the "new year for kings and festivals" (rh 1:1). Reigns of monarchs in biblical times were reckoned from that time, but later it was made the seventh month of the civil year (rh loc. cit.). The zodiacal sign of this month is Aries. In the present fixed Jewish calendar it invariably consists of 30 days, and the 1st of Nisan never falls on a Monday, Wednesday, or Friday (see *Calendar). In the 20th century Nisan, in its earliest occurrence, extended from March 13 to April 11, and in its latest from April 11 to May 10. According to R. Joshua, this is the month during which the world was created and the Patriarchs were born (rh 11a). It was in Nisan that God spoke to Moses from the burning bush. In this month redemption will occur in the time to come (ibid.). The tabernacle was erected in Nisan (Ex. 40:17), and the princes brought their offerings then (Num. 7:1–2). Because the 12 princes offered their gifts to the tabernacle every day beginning with the first of Nisan, each day was considered a festival. All public mourning is prohibited in Nisan. *Taḥanun and *Ẓidduk ha-Din are not recited, nor are eulogies allowed (Sh. Ar., oḤ 429:2). As "the greater part of the month was thus sanctified, the entire month is deemed holy" (ibid., comm. of Magen Avraham, 3).
Memorable days of Nisan include the Passover period: the 14th of Nisan, the eve of the biblical feast of *Passover when all leaven is cleared from Jewish households, and in Temple times, the *Paschal lamb was sacrificed (Ex. 12 and parallels); and the festival of Passover from the 15th to the 21st (in the Diaspora, the 22nd) of Nisan. The 15th and 21st of Nisan (in the Diaspora 15th–16th and 21st–22nd), the first and last days of Passover, respectively, are full holidays; 16th–20th of Nisan (in the Diaspora 17th–20th) are the intervening days of the festival, *ḥol ha-mo'ed. The 16th of Nisan is the controversial "morrow of the Sabbath" (see Lev. 23:11, 15, 16) when an omer of barley was offered in the Temple and marked the commencement of the counting of the omer. Other traditional dates in this month are 1st–7th of Nisan, the defeat by the Pharisees of the Sadducees' claim that the tamid (Ex. 29:38–42, Num. 28:1–8) was to be defrayed by private donations (Meg. Ta'an. 1); 8th–21st of Nisan, a Pharisaic victory over the Sadducees in a dispute concerning "the morrow of the Sabbath" and the day of the month on which Shavuot falls (Meg. Ta'an. 1); 1st (or 8th), 10th, and 26th of Nisan, the respective anniversaries of the death of *Nadab and Abihu, of *Miriam, and of *Joshua, once observed as fasts (Meg. Ta'an. 13).
Eisenstein, Dinim, 267, s.v.
[Ephraim Jehudah Wiesenberg]
"Nisan." Encyclopaedia Judaica. . Encyclopedia.com. (September 20, 2018). http://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/nisan
"Nisan." Encyclopaedia Judaica. . Retrieved September 20, 2018 from Encyclopedia.com: http://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/nisan