Spring is the best time to visit DFW.  Summer may be vacation season, but in the Metroplex the highs regularly reach or exceed 100 degree with high humidity. In Spring, especially late March and April, daytime temperatures tend to be in a comfortable 70-80 degree range.. Everything's in bloom and the weather is at its best.   Autumn is also a good time to visit.  Late September through mid-November often has pleasant days and cool nights. 

Here is the link to Dallas Weather - Five Day Forecast