By our calculations, you should look at sowing Super Sweet about 28 days before your last frost date.
From Bermuda originally. The Bermuda yellow sweet onion variety has been improved upon in Texas, and used to develop all modern supersweet varieties. 1
Domenica Supersweet, Domenica, Dominica, Dominican, Texas Supersweet, Texas 1015, 1015Y, Bermuda Supersweet, Bermuda, Bermudan, Bermudas, Bermudes, TX 1015, 1015, Ten Fifteen, Maui, Videlia, Noon, Granex
Domenica Super Sweet, Texas Supersweet 1015Y, Bermuda Yellow Sweet, Yellow Granex, Grano, Babosa, Maui Maui, Noonday, Vidalia, Georgia Sweet
1 http://aggie-horticulture.tamu.edu/archives/parsons/publications/onions/onionhis.html