Large, the sixth month of the civil and the twelfth of the ecclesiastical year of the Jews (Esther 3:7, 13; 8:12; 9:1, 15, 17, 19, 21). It included the days extending from the new moon of our March to the new moon of April. The name was first used after the Captivity. When the season was backward, and the lambs not yet of a paschal size, or the barley not forward enough for abib, then a month called Veadar, i.e., a second Adar, was intercalated.
Adar
Adar is the twelfth month of the Hebrew calendar, and the Bible names it only in Ezra and Esther. In Ezra, the rebuilt house of God was finished on the third day of Adar, in the sixth year of King Darius. In Esther, it is the month Haman’s lot fell upon, and the thirteenth day was the date set for a decree throughout the king’s provinces. The same month holds both a long task completed and a nation’s danger turned around. Esther 3 and 8 show how the story unfolds.
3 verses
Ezra
And this house was finished on the third day of the month Adar, which was in the sixth year of the reign of Darius the king.
Esther
In the first month, that is, the month Nisan, in the twelfth year of king Ahasuerus, they cast Pur, that is, the lot, before Haman from day to day, and from month to month, to the twelfth month, that is, the month Adar.
Upon one day in all the provinces of king Ahasuerus, namely, upon the thirteenth day of the twelfth month, which is the month Adar.
From Easton's Bible Dictionary
Matthew George Easton, 1897