Though it is true that Mongolia weather is considered as an extreme continental in nature but if you know and are well prepared, nothing to worry about it. First, the bad news or the worst time of the year are March and April when it gets very dry and windy, and dusty.
If you happen to travel this for any reason, expect delays from couple of hours up to 12 hours in connecting flights from Beijing, Seoul and Moscow, and from Tokyo or Osaka is you flying chatter.
The rest of the year pretty good but if you are not used to heavy winter clothes and to cold weather, it could be unpleasant. My Malaysian friend really did not like the weather in February when it was averaging minus 18-28C.
Winters are long, from November till March but it is gorgeous in the countryside. It is very smoggy in bigger towns and cities but the air is so fresh elsewhere.
The lowest temperature was recorded -55C in north-western part of Mongolia and the hottest in Gobi Desert, +40. July is the hottest month averaging 22-25 in most parts and 30-35 in Gobi desert area in the south. Due to its high altitude and landlocked position, the precipitation is very low.
The best time of Mongolia weather are September and October when it is not raining, not too cold and not too hot. You will not see greenery but you will see different colors of the nature.