The May 9th transit of Mercury begins at 7:12 a.m. Eastern Daylight Time (11:12 Universal Time), though the first contact might vary by a few minutes depending on your location..
I love these sort of questions. Behind it, there is that assumption that the expansion of mercury molecules when on DXM is a well know phenomena. To be honest, I have never heard of this before.
I tend to think that the sudden alteration of macro phenomena when tripping is more likely to be perceptual rather than real-life.
If it's looking at mercury etc in an old thermometer, I wonder if other mirror-y substances also produce that effect for you - and yes it just seems to be a perceptual thing with being unable to focus well on it since it mirrors.
If you think it's the mere presence... do a blind study with a box containing mercury and an absolutely identical box containing something else and see if you can sense the mercury not knowing in which box it is. : P