By our calculations, you should look at sowing Sweet Spanish Yellow Utah Jumbo about 28 days before your last frost date.
Utah Yellow Spanish onions are typically grown from sets that may be planted as soon as soil can be worked in the spring. They may be planted every few weeks to harvest as green onions, or may be planted one time early in the season to harvest as mature dried bulbs.
Ensure that temperatures are mild and all chance of frost has passed before planting out, as Sweet Spanish Yellow Utah Jumbo is a very hardy plant.Public Domain
Utah Yellow Spanish