Onions are a cool season crop that comes in two kinds: long day and short day. Choosing when to plant the onion sets depends not only on where you live but what kind of onion is planted. Long day onions require days of 15 to 16 hours of sunlight to bulb, which means they grow the best in northern regions. Long day onions need about 120 days to mature. Short day onions need 12 hours of sunlight to bulb; they do the best in southern regions and mature in about 85 days. Additionally, leave both kinds of onions in the ground for an additional week to two weeks, to cure.
Calculate the average last day of frost in the spring and the first day of frost in the fall. This gives you the entire time period for onions from planting in the ground to harvesting. It must be at least 85 (short day onions) to 120 (long day onions) days long. For example: if the average last day of frost is April 1 and the average first day of frost in the fall is September 15, the time period for planting and growing is 135 days long.
Plant short day onions by June 20 at the very latest in this example. The entire growing period is 135 days and ends September 15. Short day onions require 85 days for the onions to reach maturity. Counting back from September 15, 100 days brings the date to June 20.
Adjust the planting date to reflect the fact that onions require temperatures between 45 and 60 degrees to germinate and between 60 to 75 degrees to bulb and cure. Look at the average daily high temperatures in your area and plant four weeks before temperatures regularly get above 60 degrees. If the high temperature usually starts reaching over 60 degrees in late May, then set out the onion sets in early May.
Plant long day onions by April 10 since they require nearly the entire growing period of 135 days. Adjust the start date reviewing the average high temperatures. However, most of the north won't consistently reach above 60 degrees until mid-May or so.