Dallas weather varies rather drastically. The best time to visit Dallas is during Spring (March-May) and the fall (September-November), when average highs are no more than 25 degrees Celcius and the lowest does not hit lower than 15 degrees Celcius. Summers (June-July-August) could be scorching with the warmest month being august, and the winters (December-January-February) can be harsh too with occasional showers.
The Spring months of March and May are less warmer than the summer months but there's a probability of rain. May is the wettest month in Dallas, with an average precipitation of about 125 mm. Dallas sees the most footfall, surprisingly, in June-August, owing to a number of events that take place during these months. If you want to avoid the crowd then the best time to travel is during the shoulder months of Spring and Autumn. Off-season or the time to avoid travelling to Dallas is during Winters with January-February being the least favourable time.