We consider a Fermat curve Fn : x n + y n + z n = 1 over an algebraically closed field k of characteristic p ≥ 0 and study the action of the automorphism group G = (Z/nZ × Z/nZ) ⋊ S 3 on the canonical ring R = H 0 (Fn, Ω ⊗m Fn ) when p > 3, p ∤ n and n − 1 is not a power of p. In particular, we explicitly determine the classes [H 0 (Fn, Ω ⊗mFn )] in the Grothendieck group K 0 (G, k) of finitely generated k[G]-modules, describe the respective equivariant Hilbert series H R,G (t) as a rational function, and use our results to write a program in Sage that computes H R,G (t) for an arbitrary Fermat curve.