(This should not be relied upon for practical halacha. When a question arises a Rabbi should be consulted.)
1, The Shulchan Aruch (Even Haezer 65:3, the source is a Gemara Baba Basra 60b) writes that we must place ashes on the chosson’s head as a sign of mourning over the destruction of the Temple. The ashes are placed on the same area of the head that the chosson wears his tefillin.
2. The Kaf Hachaim (Orach Chaim 660) notes that the custom in his area was not to place ashes on the chosson’s head (see also Orchos Chaim Tisha B’Av 13).
Harav Ben Tzion Abba Shaul zt”l (Ohr Letzion vol. 3 page 277) also notes that many sefardim do not place ashes on the head of the chosson and wonders how they can disregard this custom, which has sources in the Gemara and the Shulchan Aruch. He therefore rules that even sefardim should participate in this custom.
Harav Ovadia Yosef zt”l (cited in Yalkut Yosef Sovea Smachos page 86), however, rules that sefardim should continue in their practice of not placing ashes on the chosson’s head.
3. The custom of Square Chassidim is also not to place ashes on the chosson’s head (Sefer Netai Gavriel Nisuin page 117).
4. The Taz (Orach Chaim 660:4) writes that some have the custom that as the ashes are being placed on the chosson’s head, the rabbi recites the verse “Im eshkocheich Yerushalayim etc.” The chosson then repeats the verse.
5. The overwhelming majority of poskim maintain that the ashes should be placed before the chosson enters the chuppah. However, Harav Yosef Dov Soloveitchick zt”l (Rosh Yeshiva of Yeshivas Rabbeinu Yitzchak Elchonon) would only place the ashes on the chosson after the birchas nisuin were recited under the chuppah. Before the chuppah ceremony the chosson is not yet considered a “chosson” in Jewish law, and there is not yet a requirement of ashes. After the chuppah ceremony he is considered a “chosson” and the ashes are then required (Journal Mesorah vol. 8 page 52). It seems that Harav Soloveitchick zt”l shared the same view of his uncle, the great Brisker Rav. As the Sefer Yismach Lev (page 76) reports, the Brisker Rav would only place the ashes after the birchas nisuin under the chuppah, for the very reason attributed to Harav Soloveitchick.
6. The Aruch Hashulchan (Even Haezer 65:4) feels that the ashes are removed immediately after their placement. However, the Shulchan Haezer (7:1:11) writes that the custom in his town was to leave the ashes on the chosson’s head. A similar view is expressed by Harav Shlomo Zalman Auerbach zt”l (Shalmei Simcha page 169 – ועיין שם דבר חדש בדף קס”ט: “ורבינו היה מורה ליתן את האפר על ראש החתן כשהוא עטוף בנייר, וזאת משום כבודו של החתן, וכשסידר את הקידושין היה הוא עצמו מניח את האפר עטוף בנייר על ראש החתן -).
7. Harav Auerbach zt”l adds that if they forgot to place the ashes before the chuppah they may do so after the chuppah.
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