What's stopping them from taking off at night at Xian, landing it at a secret airbase in the desert then flying it back to Xian the next night and store the aircraft in a hangar during daylight hours? The Roar of 4x WS-18 at night at Xian's AC will not be unusual at night either and it wouldn't raise any suspicions that it would be a strategic bomber taking off instead of a Y-20 transport.
Maiden flight tests are always done in daytime, primarily for safety and visibility reasons.
During maiden flight of any brand new warplane, you want the pilots of the chase plane (typically a twin-seater fighter) to be able to visually monitor the new warplane's behavior and performance during the flight, and be able to spot anything out of the ordinary, especially in (a lot of) places of the new warplane where the test flight pilots are unable to see themselves. This also ensures that safer and proper emergency responses can be conducted in time in case something goes wrong during the flight.
Basically, you want another pair (or two) of eyes to be on the lookout to make sure that the new warplane can do everything any other warplane does (takeoff, fly, and land) before proceeding any further.
Moreover, there should be a camera mounted on the chase plane to photograph and videotape the new warplane during its maiden flight, of which the photos and video obtained will be used for post-flight analysis and study.
Such abilities will be severely hampered during nighttime.
Also, you want the test flight pilots to be able to observe things clearly during the maiden flight in order to be able to properly respond to any changes to the new warplane itself by eliminating as much foreign-introduced uncertainties as possible, especially when the new warplane is already full of uncertainties itself. Flying at night will only introduce more uncertainties due to poorer visibility, which risks making things worse if something goes wrong during the maiden flight test.
This is not to say that there won't be nighttime flight tests for the H-20. It will be done in the latter stages of the development of the new warplane, particularly when all the fundamental features and flight performances of the H-20 has been conducted and verified.