מדרש אויף אסתר 3:7

"This month": Nissan. You say it is Nissan. But perhaps it was some other month of the year? It is written (Exodus 23:16) "And the festival of the ingathering (Succoth) at the end of the year," (Ibid. 34:22) "And the festival of the ingathering at the turn of the year." In which month is there ingathering and turning (from one solstice to another) and the expiration of a year? Tishrei. After learning that the seventh month is Tishrei, we infer that the first month is Nissan. And even though there is no proof for this, it is intimated in (Esther 3:7) "In the first month, the month of Nissan."

(Lev. 25:25:) “When your relative becomes poor.” This is Israel, of whom it is stated (in Ps. 106:43), “so they became poor through their iniquity.” (Lev. 25:25, cont.:) “And sells some of his property,” in Media in the days of Haman. (Lev. 25:25, cont.:) “Then his redeemer shall come.” This is Mordecai. (Lev. 25:25, cont.:) “And redeem what his relative has sold,” in that he covered the sins of Israel, since they all were deserving of slaughter, because they had eaten some cooked food of star worshipers.²⁴See Esth. R. 2:5. It is so stated (in Esth. 1:5), “And when these days were fulfilled, [the king made a seven-day banquet for all the people that were to be found in Shushan the capital, both great and small].” Now Haman tormented them (i.e., the Jews), as stated (in Esth. 3:7), “a pur, i.e., the lot, [was cast before Haman…].”²⁵The previous verse indicates that the lot was being cast to determine the best time to move against all the Jews of the kingdom. But due to the merit of Mordecai they were delivered, as stated (in Esth. 9:1), “the opposite happened, [in that] the Jews gained control over their enemies.” Ergo (in Lev. 25:25, cont.), “and redeem what his relative has sold.”Another interpretation (of Lev. 25:25, cont.), “then his redeemer shall come”: This is the Holy One, blessed be He, since it is stated (in Jer. 50:34), “Their Redeemer is mighty, His name is the Lord of hosts.” (Lev. 25:25), “The one most closely related (qarov) to him.” This is the Holy One, blessed be He, since it is stated (in Ps. 148:14), “He has raised up a horn for His people, praise for all His saints, even the Children of Israel, a people near (qarov) to Him. Hallelujah”.

This month shall be unto you (Exod. 12:1). R. Ishmael said: He showed him the moon during night and said: Like this shall you examine the new moon; this shall be the law for future generations. Teach them the time of the new moon. He said to them: Until now I intercalated the year (adding an additional month in leap years), but now I turn this responsibility over to you. From now on you shall count (the days and designate the time) for the appearance of the new moon. This month. This is Nisan, and there is no other (i.e., no intercalated second Nisan to sere as the first month). With regard to Tishri it is written: And the feast of ingathering (Sukkot) at the turn of the year (the autumnal equinox) (Exod. 34:22). This teaches us that the month in which the festival called the feast of the ingathering at the turn of the year occurs is the month in which the year actually changes. However, Tishri is called the seventh month only because it comes seven months after Nisan. This month refers to Nisan. A hint of this (is the fact that it is the only month that is spoken of as the first month),⁷Nisan begins the secular year and therefore is counted as the first month. It marks the anniversary of the deliverance from Egyptian bondage. It is the month in which the reigns of kings begin, and the month from which the festivals are instituted. as Scripture states: In the first month, which is the month of Nisan (Est. 3:17), first for months, and first for festivals, since (Passover) is written first: The festival of Passover, the festival of Shavuot, the festival of Sukkot.