2 presumed Mongolian Gulls? on the same date.
But not I could not observe them enough,

A sleeping bird on the left is the bird.
The wing-molt was finished.  Its new P10is seen in the underside of the far wing. This molt was exceptionally early among the large flock.
 


2 new primaries are seen here. It would be a relatively later example for mongolicus. But other points look suitable for mongolicus.