Mongolia has a continental climate with extreme low temperatures in winter (up to -40°C or even lower) and extremely high temperatures in summer (up to +40°C). Of course, the weather varies a bit from region to region.
So when is the best time to visit Mongolia?
I was there in August and it was great. However, July seems to be the great period because of the Naadam Festival; on the other hand it may be too warm for enjoying the Gobi desert.