Q.  How come that the Mishenichnas Adar Marbim Besimcha mitzva, meaning that when Adar begins we have to increase our happiness is so undefined?
Firstly which Adar does it apply to, when as in this year there are two? When Rosh Chodesh is two days, on which day does it begin? Does it start from the Molad of the month when it is earlier? And why is it that according to some even from Tu Bishvat, Why so much uncertainty?
A. Regarding the practical differences when Marbim Besimcha or when Adar actually begins, on question 2045 we mentioned undergoing an important medical procedure. On 472 we dealt with court and legal cases.
On question 3049 and 1634 we mentioned that Adar is a time when good Mazal prevails, so it is a propitious period to begin new shiurim, establish new mitzva projects, and other endeavors whose success will bring joy. (See question 990 regarding new business ventures).
On question 990 we added: “Derech Sicha, (Miketz p. 188,) Nitey Gavriel, (Purim – Teshuvos 2) et. al., opine that it begins from the first Adar. Nitei Gavriel (ibid. 464) quoting the Munkatcher Rebbe, adds even from Tu Bishvat.”
The reason why the beginning date and time is so undefined and not exactly and clearly established, may be that likely it depends on the nature of each individual and his particular needs and situation. The great month of Adar is willing to accommodate itself to the needs of each individual.
Rabbi A. Bartfeld as advised by Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu Shlit’a