VISA FOR MY ROUND TRIP

We are planning a round trip from Beijing to Mongolia, Irkutsk in Russia and back to China. We have air fares from 6.7. 2013 to 4. 8. 2013. We are staying in Beijing till 9th JUly, then we',, go to Mongolia for two weeks, continue to IRkutsk and at the end of july back.
I'd like to ask about it and when to come to apply for it.
Thanks,
Marija Vranicar