Red Cardinal is great from sowings in either February or early August. Sowing in April and May results in early flowering, on all spinach and especially the red varieties.
I am not sowing any more spinach now until late July. Next year see if you can either propagate or buy a few early plants of eg Boa, Mississipi, Toscane – afraid they are all hybrids but they give leaves for a long time, I am picking some beauties at the moment (from March sowing in fact).