(This should not be relied upon for practical halacha. When a question arises a Rabbi should be consulted.)
The Gemara in Pesachim (113b) states that there are seven types of people who are banned (excommunicated) by Heaven. After describing each type the Gemara adds, “Some say, also one who does not eat at a meal celebrating a mitzvah.” Tosafos explains that the Gemara is describing one of three cases, those who do not eat at the meal accompanying a circumcision, a wedding of a scholar or the wedding of a kohen who marries a bas kohen.
The Rama codifies this teaching in the laws of circumcision (Yoreh Deah 265:12). He writes that, “One who does not eat at a circumcision meal is as if he is excommunicated by Heaven.” The Pischei Teshuva (265:18) comments that since many people may not be able to attend it is better not to publicly invite everyone to the circumcision meal and spare them of the punishment listed in the Gemara.
One may argue that the same stringency be extended to wedding invitations and that if one is invited to a wedding he must attend. Indeed, the Sefer Chupas Chassanim (Seuda note 10), based on the above sources, advises not to invite someone who will (most likely) not attend the wedding.
However, the very widespread practice today is not to attend every wedding that one is invited to. In many cases people invite those who may not be able to attend (out of town relatives etc.). I believe that based on the words of the poskim, one can offer many defenses for the common practice of the observant community to not attend every wedding that they are invited to.
1) Harav Gavriel Zinner shlit”a writes that sending a wedding invitation does not necessarily mean that you are inviting them to the wedding. Many times the invitation is used to inform others about the marriage that will be taking place. The proof of this, he writes, is the fact that one sends invitations to people outside of the country who will clearly not come to the wedding. In this scenario the invitation is more of a formality than an actual request of their presence (Netai Gavriel Nisuin page 90. See also Shulchan Haezer vol.2 Page 68 and Koveitz Ohalei Shem vol. 5 Page 32).
2) Harav Moshe Feinstein zt”l notes that while Tosafos writes that the excommunication (discussed in the Gemara) applies to weddings as well as circumcisions, the Rama only codifies this law in the laws of circumcisions. Therefore, writes Rav Moshe, the Rama maintains that this law does not apply to weddings (Igros Moshe O.C. 2:95). [It should be noted that Harav Moshe himself would do all that he could to attend every wedding that he was invited to, even if that meant attending numerous weddings in one night (Oral ruling of Harav David Feinstein shlit”a cited by the Sefer Shalmei Simcha page 312).] Harav Chaim Kanievsky shlit”a also rules that the ban only applies to circumcisions and not weddings (Sefer Yismach Lev 50).
3) Harav Shlomo Zalman Auerbach zt”l maintains that if one is present at the chuppah he is required to remain at the wedding throughout the entire ceremony. One is not required to attend the wedding simply because he knows where and when a wedding will take place (Shalmei Simcha Ibid.). A similar notion is expressed by the Sefer Yismach Moshe (See Sefer Yismach Lev page 37).
4)Harav Moshe Shternbuch shlit”a writes that there is only a prohibition if one does not attend a wedding because he feels that he is too honorable to attend and that it would be beneath his dignity to remain at a wedding with people “beneath” his character. If one cannot attend the wedding for other legitimate reasons, there is no prohibition or ban (Teshuvos V’Hanahagos 2:649).
5) The Kaf Hacheim, citing the Sefer Yafeh Lelev, writes that one is only obligated to attend the circumcision meal if there is less than ten men in attendance. If there are more than ten men in attendance, one may skip the meal. The same can be applied to weddings and if more than ten men are in attendance, (which is always the case), one would be allowed to not attend the wedding (Kaf Hachaim 170:71).
6) Tosafos writes that one is not obligated to attend a wedding meal if men who are unethical or improper will also be in attendance. Some poskim argue that today the average wedding is attended by people who fall under this category (improper) and thus one is never obligated to attend a wedding (See Yabia Omer Y.D. 4:19).
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