There is an ancient custom, recorded in the Talmud (Bava Basra 60b, later quoted in the Shulchan Aruch, Even HaEzer 65:3) of placing ashes on the head of the groom, in the place where tefillin is usually worn, as a way of fulfilling the verse “If I do not place Jerusalem on the head of my rejoicing.”
There are communities in which this custom is not practiced, including many Sephardic ones. Some Chassidic communities and some latter-day Sephardic authorities wonder why the practice fell into desuetude. It may be related to the lack of widespread observance of tefillin for a long time in many communities. Fortunately, tefillin observance is now more widespread, and the practice of placing ashes has become more common, too. (Rav Soloveichik, zt”l, did so assiduously under every chuppah after the first two blessings).
I do so whenever I have the honor of officiating at a wedding. While taking care not to get the chosson dirty, it is a tangible reminder for the new couple that as much as they are building a new world together, they are doing so in a world that is still broken. Life is beautiful, but it surely is bittersweet, and their success as a couple will rest on their support of one another through the chuppah –and the ashes – of life.