The best place to look up UAE's weather data is at Weather.com

http://www.weather.com/

From that link, it is possible to deduce that the most comfortable times of year to visit are between October and April.  June to mid-September are fiercely hot with high humidity.  Desert trips at this time, although lacking humidity, are likely to be in mid 50s C and can bring about heat exhaustion in a very short time.