When to start directly outside (based on frost date): May 4-18
If you are thinking of planting the "Three Sisters" remember they're sisters, not triplets- plant them according to their own dates

Photo by Bicanski at https://pixnio.com/flora-plants/crops/green-young-corn-on-the-stalk-in-field
Scientific Name: Zea mays
Varieties (not a complete list): Glass Gem popping, Orchard Baby Sweet, Buhl Sweet
Colors: Yellow, white, multicolored, and more
Types: Sweet, Flint, Popping
Sprouts in: 7-10 days
Ideal Temp: 75-80F
Seed Depth: 1 to 2"
Plant Spacing: 12" apart
Frost Hardy: No
Min Full Sun: 6-12 hrs
Planting Instructions: Direct-sow into the richest soil available 1-2 weeks before last spring frost. Succession-planting prolongs harvest; planting in blocks improves pollination.