Downtown Weslaco. Free admission & Parking. (956) 968-2102. The Texas Onion Fest celebrates the development of the world famous Texas 1015 Onion which was developed in Weslaco. The name of the 1015 onion came from the suggested planting date, October 15. Texas Onion Fest is sponsored by the Weslaco Area Chamber of Commerce. Live music, performances, Health fair-Market. Culinary demos, kiddie rides, onion blossoms and much more!!