Hi, is it still ideal to visit Xilingol in 2nd week of Sept?

Well, generally speaking, July and August are two best months to visit there. The second week of September already turns cold and scenery is not that beautiful. If possible, you can change your travel plan.

Where in Xilingol are you going to, the Xilinhot city? If yes, there is no direct long-distance, train or flight that can take you to the destination. You can go to Hohhot by train first (6.5 hours), then take a long-distance bus to Xilinhot (about 7 hours).