Do it now. Trust me. Do it before the first snow, planted any way you like; broadcast atop the soil, buried slightly in rows, it doesn’t matter. Spinach will spend the winter getting ready to burst forth in great profusion in spring. And I mean GREAT profusion.
By comparison, spinach I sow in spring — no matter how early — does, well, pretty much ahhh. Marginal.
I don’t have a good photo of spinach because we ate it all.