Spinach is one of the first things you can eat out of your garden. They can be sown as soon as the ground is workable. Mild frost would not hurt them. I usually sow my spinach the same time I plant my onion sets. This year, to experiment with inter-cropping, I sowed my spinach in between my rows of onions. The result is great. Spinach grow fast, they cover the ground when onions are still small-saves space and suppresses weeds. It is a win-win!
When the spinach is 3-4" tall, I just cut the leaves with a pair of scissors and let the plant grow more leaves.