  A cyclic descent function on standard Young tableaux of size $n$ is a
function that restricts to the usual descent function when $n$ is omitted, such
that the number of standard Young tableaux of given shape with cyclic descent
set $D\subset[n]$ is invariant under any modulo $n$ shift of $D$. The notion of
cyclic descent was first studied for rectangles by Rhoades, and then
generalized to certain families of skew shapes by Adin, Elizalde, and Roichman.
Adin, Reiner, and Roichman proved that a skew shape has a cyclic descent map if
and only if it is not a connected ribbon. Unfortunately, their proof is
nonconstructive; until now, explicit cyclic descent maps are known only for
small families of shapes.
  In this paper, we construct an explicit cyclic descent map for all shapes
where this is possible. We thus provide a constructive proof of Adin, Reiner,
and Roichman's result. Our construction of a cyclic descent map generalizes
many of the constructions in the literature.
