The notion of descent set, for permutations as well as for standard Young tableaux (SYT), is classical. Cellini introduced a natural notion of cyclic descent set for permutations, and Rhoades introduced such a notion for SYT -but only for rectangular shapes. In this work we define cyclic extensions of descent sets in a general context, and prove existence and essential uniqueness for SYT of almost all shapes. The proof applies nonnegativity properties of Postnikov's toric Schur polynomials, providing a new interpretation of certain Gromov-Witten invariants. Date: November 22, 2017. Proof of Lemma 2.2(i). The nonnegativity property (a) is obvious, with m(∅) = m([n]) = 0 following from the non-Escher axiom, while properties (b) and (c) are consequences of the equivariance and extension axioms in Definition 2.1. Proof of Lemma 2.2(ii). Let (m(J)) J⊆[n] be integers satisfying conditions (a),(b),(c) above. For each J ⊆ [n − 1] choose, arbitrarily, a subset T 0 J of T J := Des −1 (J) of size #T 0 J = m(J), and denote T 1 J := T J \ T 0 J ; by condition (c), #T 1 J = m(J ⊔ {n}). By construction, T = J⊆[n−1](T 0 J ⊔ T 1 J ).Define cDes : T −→ 2 [n] by