Q: May ashes be distributed on days other than Ash Wednesday?
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A: The best I can tell you, “There is no provision for it.” I don’t see anywhere that it is forbidden, but all of the references tie ashes to the Wednesday that precedes the first Sunday of Lent.
Not only the liturgical books, but liturgical history and common custom.
There are probably circumstances when someone does distribute ashes on a different day – mission churches at a considerable distance, prisons with limited access, for example. But the liturgical books do not envision those exceptions.
The Book of Blessings for the United States of America includes an Order for the Blessing and Distribution of Ashes apart from mass (chapter 52), but it too is envisioned for Ash Wednesday.