Zone doesn't tell you anything except the expected coldest temperature each year, which is in winter, so that doesn't matter.

My first light frost is usually around September 15. I try to have my last crop of peas in by August 1. They're in now and above ground. Peas are hardy and will survive a little snow.

Spinach germinates better when the soil temp is below 80°.

 

Check the variety days from planting to harvest and add a week or two, then count the days back from the first frost expectation date. Be prepared to cover them when the frost hits, then enjoy fresh spinach through Indian summer.

You can also sow spinach in September, and when it is a couple inches high, over sow with oats. The oats will winterkill and provide mulch protection. The spinach will resume growth in the spring.