Well, that's what you say...
But Wiktionary (without a citation, so pinch of salt time) says that it's an Old Irish borrowing from Latin septimana, which comes from the Latin for seven.
So although seachd and septimana have cognate roots, seachdain apparently isn't from seachd.
EDIT: the derivation via Latin is given in this 1910 Gaelic etymological dictionary.
Oh, wow, you’re right. Didn’t expect that.
But then, the words were still so close in the time of Primitive Irish (when it was borrowed, and that early Prim. Ir. lacked /p/ and borrowed it from Latin as /k(ʷ)/, lenited to /x/, compare Lat. Pascha → Ir. Cáisc, Sc. Gaelic Càisg) that it shouldn’t surprise that a Latin borrowing looks like a derivation of a native word. Interesting nonetheless.
Thanks for verifying it. :)