Adar

large, the sixth month of the civil and the twelfth of the ecclesiastical year of the Jews (Esther 3:7; 3:13; 8:12; 9:1; 9:15; 9:17; 9:19; 9:21). 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.

EBD - Easton's Bible Dictionary