The climate is warm throughout the year in Riyadh -- and quite oppressively hot during the summertime. The average temperature during the winter time is in the upper 50s to mid 60s, while average temperature in the summertime is a balmy mid to upper 90s. Average highs in the winter are in the upper 60s to low 70s, and lows average in the upper 40s to the low 50s. During the summer, high temperatures average in the upper 110s, and low temperatures average in the low to mid 80s. Rain is relatively nonexistent throughout the year. With the majority of precipitation (little though it may be) occuring during the late spring months. 

With little to no precipitation and still rather warm temperatures, the best time of year to visit Riyadh may be the late winter.