The verse in Tehillim (Chapter 137) states: “If I forget you, Jerusalem, let me forget my right hand; let my tongue stick to my palate if I do not remember you, if I do not mention Jerusalem at the peak of my joy.” The Gemara in Masechet Baba Batra (60b) states that “at the peak of my joy” refers to the ashes placed on the head of a groom on his wedding day in commemoration of the destruction of the Bet Hamikdash. Indeed, many Ashkenazi communities observe this custom until today.
This custom is mentioned by the Rambam and Maran Ha’Shulchan Aruch as follows: “When the groom marries his bride, he must take ashes and place them on his head in the place where the Tefillin are laid, as the verse (Yeshaya 61) states, “To place for the mourners of Zion glory instead of ash” and Tefillin are referred to as ‘glory’.” The Rif and Rosh rule likewise.
Maran Ha’Bet Yosef quotes the Kol Bo who writes that there are certain places where this custom is not observed, for they are concerned that the Tefillin of this groom may not be valid, in which case we are concerned that just as he has not fulfilled the edict of “ash instead of glory” now, he will likewise not merit the promise of “glory instead of ash” in the future. An alternative custom is observed though where the groom breaks a glass after the Seven Blessings are recited under the Chuppah (wedding canopy). This is indeed our custom, whereas the groom breaks a glass under the Chuppah in commemoration of the destruction of the Bet Hamikdash.
Rabbeinu Yosef Tarani, in his Sefer Tzafenat Pane’ach, writes that the reason why specifically a glass is broken is in order to hint that just as glass can be repaired after it has been broken by melting it down and forming a new cup, so too, the Jewish nation can likewise be “repaired” by Hashem redeeming them eternally.
Maran zt”l points out on this topic that nowadays, a new baseless custom has emerged which is that at the time the groom breaks the glass, joyous shouts of “Mazal Tov” erupt from those assembled. Sometimes, even the groom tries to break the glass with all his might childishly. Thus, the true meaning of this beautiful custom which is intended to show agony over the destruction of our holy Temple and raise Jerusalem to the peak of our joy has been changed to a trivial custom of jesting and light-headedness. Other great Poskim have already pointed this out. It is therefore appropriate that the groom recite the verse “If I forget you Jerusalem” before breaking the glass and in this way, people may slowly realize the intended message of this age-old custom.