Q. Does Mishenichas Adar Marbim Besimcha (when the month of Adar begins we increase joy), apply also after Purim? Does the joy keep on increasing? Does it continue into Nissan too? (for the purpose of starting a new business or scheduling a medical operation that can be postponed)

A. Although the mitzva of being besimcha and rejoicing in serving Hashem and performing His mitzvos applies always (Rambam H. Lulav 8: 15, Rema end of O.H.)), there is an additional commitment to do so in the month of Adar. .
Eishel Arohom (2: 685: 8) asserts that it includes the complete month of Adar, as the posuk (Esther 9: 22) reads “the month which was turned for them from sorrow to joy.” See similar Chasam Sofer (hag. O.H. 551: 2).
Shaarei Hamoadim (p. 20) maintains that the simcha increases from day to day and even from hour to hour.
Rashi (Taanis 29a) explains that the marbim besimcha is due to the days of miracles of Purim and Pesach. Therefore, Eliahu Rabba (685: 8), Shearim Metzuyanim Behalacha (141: 1), and others affirm that it extends even to the month of Nissan. See also Nitei Gavriel (Purim 11: n. 3) as to the view of the Chassam Sofer and Sheilas Yaavetz.
Horav Shlomo Miller’s opinion is that if someone can choose to have a medical procedure done during the beginning of Nissan, if it will not interfere with the enjoyment of the Yom Tov, he should do so. He also maintains that the marbim besimcha ends at the end of Pesach, when the Sefira days begin.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a