Onions are generally grown from sets planted in early spring. Growing Onion from seeds is another option and generally gives you a much more diverse array of varieties to choose from. Onion Seeds can be sown in the garden in the midfall to over winter and produce a crop next season. They can also be planted as 'late' or as 'early' as late January, depending on how you look at it. You'll need a warm bright area to get them going if starting in mid to late winter.
Onions sown from seed are not as likely to bolt than those grown from sets so they may be worthwhile if Spring temperatures are erratic. Onions are biennial, they have a two year growth cycle before they ‘go to seed’. Onion sets are immature onions grown the previous year, they're already yearlings that are easily tricked into thinking another year has passed if spring temperatures shift from warm back to cold. If you grow onions from seed this is rarely a problem.
Planting them directly in the soil in the spring is only advisable in the best of climates otherwise they should be seeded into cell pots or a suitable container for transplant into the garden in spring.
Sow onion seeds about 1/4 inch deep, covering lightly. Most onion seed varieties should germinate in approximately 2 weeks. Grow onions in individual peat pots or cell packs to reduce transplant shock, several plants per cell or pack are advisable. A relatively cool location with full sun for optimal results.
Onion sets can be planted directly into the ground in very early spring, as soon as the soil is workable. Spacing and depth varies among varieties but common sense dictates that you anticipate the full size of the onion plant, whatever variety you are planting, and leave adequate space for the bulbs to develop. If you are a tad off the optimal spacing - beware the sky is gonna come crashing down and the Universe is now out of synch.