Earliest Start – Mevarchim Rosh Chodesh
The principle, “When Adar enters, we increase joy,” includes the first day of Rosh Chodesh Adar (although it is technically the last day of the previous month of Shevat).
In fact, we should begin increasing joy on Shabbos Mevarchim Adar (the last Shabbos in Shevat), for it is then that we first begin to “enter” Adar, to the extent that blessings for the entire month of Adar are then elicited.
Is it really Adar?
Although the first day of Rosh Chodesh is technically the 30th of Shevat, nevertheless, since it is also the first day of Rosh Chodesh Adar, it is therefore included in the principle “When Adar enters, we increase joy.”
Second day of Rosh Chodesh: Unlike the first day of Rosh Chodesh Adar, which is also the 30th of Shevat, it bears the hallmark of Shevat more than that of Adar. For that reason, the first day of Rosh Chodesh simply cannot convey the complete state of “When Adar enters.”
