Texas spans zones 6b to 10a, so “when to plant tomatoes” has no single answer. What holds statewide is the two-season structure: a spring crop and a fall crop, with a dead zone in between.
The heat problem
Tomatoes stop setting fruit above about 92°F daytime and 75°F overnight. In most of Texas that arrives in June, which is why the spring crop must be in the ground early enough to fruit before it.
Spring timing by region
South Texas: mid-February. Central: mid-March. Panhandle: late April. Count back six weeks from those dates for seed starting.