When to plant onions
There is an old saying that onions, like garlic, should be planted on the shortest day (in winter) and harvested on the longest day (mid-summer). This saying is a good guide, if you live in a climate with milder winter temperatures (little or no snow).
In reality, onions can be planted anytime during winter, if you live in an area where night time temperatures don't get more than a couple of degrees below freezing. Onions are frost-tolerant (or hardy), meaning frost won't kill them. However, very low temperatures will stunt their growth.
If you live in a colder area, then early to mid-spring may be the best time to plant. Onions mature heading into summer, and need a good run of warmer weather towards the end of their life to fatten properly.
Follow the region specific advice on the back of the seed packet, if it is there. However, this advice is usually very general and should be seen as a guide only.
How to plant onions
The easiest method with onions is to raise in seed trays first and then plant out into the garden when they are about 10cm (4 inch) tall. Use a good seed raising mix, scatter some seeds and then sprinkle more seed raising mix on top until the seeds are just covered.
When planting the young seedlings out in the garden, first prepare the soil. A good way to plant onions is to use a pencil or straight stick to create the hole, then drop the plant in (roots first of course) and firm the soil gently around the base of the plant. Be careful not to plant too deep. Aim to plant at a depth where the soil is at the same level as when they grew in the tray, or marginally deeper (you don't want them falling over).
Alternatively you can plant onion seeds directly into the garden. Prepare the soil, create a small trench (less than 1cm or 1/2 inch deep), scatter the seeds and cover. Scatter more seed than you think you will need, then when plants are large enough thin out to about 12cm (5 inch) apart. Thinnings can be used in salads as spring onions.
One way of telling when onions are ready to harvest is when the tops begin to yellow slightly and/or flop over. However, this method is not 100% reliable. Because onions don't have to ripen as such, you can harvest as soon as they get to a usable size.
If you have planted your onions in mid-winter, then they are likely to be ready in mid-summer. If you planted in spring then you will probably be waiting until late summer or early autumn.
Store onions in a dry, well-ventilated place. Leave the tops on if you can. The tops will dry out and onions with their tops left on can store for several months depending on the variety.