Whether you can plant now all depends on your location, your average climate as defined by your gardening zone. You don't include that info.

Air temps are basically irrelevant until AFTER the plants emerge. It is soil temps that determine whether seeds will germinate or just lie there and rot while waiting. Spinach needs a minimum soil temp of 40-45 degrees to germinate. Then once the plants emerge you will have to protect them from such hard freezes. They will tolerate cold but not 20 degrees.

So look up your zone for your specific location and then Google for 'average planting dates for zone ______".

Dave

Thanks