The Mechaber paskens (696:8) that one may hold a wedding on Purim. The Magen Avraham poses the obvious question: One is not permitted to hold a wedding on a Yom Tov because “ain m’arvin simcha b’simcha,” one cannot mix two different celebrations together. Since Purim is also a day of simcha, why are weddings allowed?
There are two distinct elements to simchas ha’yom: 1) the nature of the day itself; 2) the chovas hagavra to perform acts that reflect that simcha. R’ Betzalel Zolti (Mishnas Ya’avetz #79) explains that both of these elements are present on Yom Tov. Simchas Yom Tov is part of the character of the day; the chovas ha’gavra to eat shalmei simcha or read hallel is an expression of that underlying character of the day. Not so the simcha of Purim. Purim is a day of “mishteh v’simcha” – simcha is not a defining quality of the day, but is connected specifically to the chovas hagavra of seudah.
The Rambam writes (Hil Yom Tov 6:18) writes that simchas Yom Tov is observed by giving little children nuts [candy?], buying nice clothers for our wives, etc. Yet, in Hil Purim the Rambam makes no mention of these obligations. He refers only to fulfilling mishteh v’simcha in eating seudas Purim. Why does the simcha of Purim not warrant the same as the simcha of Yom Tov? Because Purim is not a day of simcha -- it is day in which we have a chovas ha'gavra to eat a festive meal characterized by simcha. For this reason the Rambam mentions the obligation to drink wine on Purim only in the context of the seudah, to the exclusions of the rest of the day.
The principle of “ain m’arvin simcha b’simcha” means that one cannot schedule a wedding on a day which is already characterized as one of simcha. Purim is a day where there is a chovas hagavra to have a mishteh, but not one in which simcha in not an inherent quality of the day.
Maybe more on this topic (esp. how it relates to the halachos of aveilus on Purim) in a future post.