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. Chances are that your flights will be delayed due to gusty winds in the capital city. Foreign airlines are very cautious but the domestic ones still operate the flights.
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 pollution is off the chart in the capital city of Ulaanbaatar mainly because rapidly expanding traditional ger areas which uses coal. Also, the city is over crowded. It was built for half a million people but now it has three times more people.
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. There are some changes with the global warming. Sometimes, we have seen unusual warm during the supposedly cold times.
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. Expect to be chilly in the mornings, sometime even frost in the morning but it will quickly melt away with the sun. Some days can become quite warm. We call them "Autumn sunshine" because we feel sun is giving the best before the long and cold winter. At the same time, it can become quite cold during the nights. It can snow anytime but it will usually start with late rains which will quickly turn into snow.