AAOK is exactly right, although calves have been getting a little bit younger and doing well in Houston. For Fort Worth, you really need a big mature calf which requires a May-July calf to really have a chance to do well. Calves at Fort Worth are usually pulled back harder than others.
August-Oct calves are better for Houston. You'll do fine with September and early October calves as long as you don't have any hickups.
12-13 month old calves might get heavy enough to make minimum weight by pushing really hard, but they won't be matured and finished. We are feeding an early January calf for Houston this year for a lightweight Angus for my young son. I've never had a calf in the barn nearly that young in 20 years of showing.