Please be aware that Beijing has multiple airports and that these results are for all airports serving the city.
The best month to fly between Beijing and Xi an is February and the most expensive month to fly is January. (Average prices, based on 657 datapoints.)
The best day to arrive at Beijing is Monday and it would be best to avoid Thursday, as prices are on average 85.28% higher than on Monday. (Average prices, based on 463 datapoints.)
The best time to fly is at around 22. (Average prices, based on 668 datapoints.)