Planting time depends upon where you live. In California, I planted them in the fall. In Reno, I plant them in the spring. Your local onion garden center will sell them at the right time for planting.
They will also sell onions approriate for your area (Short day vs long day).
https://garden.org/learn/artic...
Leave about 4 inches between plants, assuming that your onions will end up 4 inches wide. Six inches if you think they will be monsters.
