Transits of Mercury don't happen very often. The last was on May 7, 2003, and the next doesn't come until May 9, 2016.
Well-prepared observers will see Mercury edge onto the Sun's face around 2:12 p.m. Eastern Standard Time (1:12 p.m. Central, 12:12 p.m. Mountain, 11:12 a.m. Pacific). Watchers in western North America can see the entire transit, which ends when Mercury slips off the Sun's edge at 4:10 p.m. Pacific time. Farther east, the Sun will set while the transit is still in progress.
To see the event, you'll have to be Sun-safe! You can burn a permanent blind spot in your eye's retina by trying to observe the Sun without proper protection — and a telescope makes the danger more intense. You'll need a safe solar filter designed to go over the front of your telescope. Alternatively, you can project the Sun's image out of the eyepiece of a telescope or binoculars onto a white card held a foot or two away; turn the focus knob to get a sharp image of the Sun on the card. For details and illustrations see "How to Watch a Partial Solar Eclipse Safely."
What to Expect
What will you see? Mercury is remarkably tiny compared to the Sun, appearing only 1/200 as wide during this transit. Even so, the little planet is big enough that in a safely filtered telescope, the total blackness of its silhouette can be distinguished from the dark gray of any sunspots that may be present. About 3/4 of the way through the transit (around 3 p.m. Pacific Standard Time; 11h UT) Mercury should pass very close to Sunspot 921 if the spot still survives as of Wednesday.
The transit begins at first contact. This is when Mercury's leading edge first touches the Sun's southeastern edge. The time differs by a couple of minutes depending on where on Earth you're looking from. (For exact predictions at many locations, see NASA's Transit of Mercury website.) Second contact, when Mercury's trailing edge enters the Sun's disk, comes slightly less than two minutes later.
Deepest transit occurs around 4:41 p.m. EST, when Mercury is a little closer to the Sun's center than edge. The transit ends within a couple minutes of 4:08 p.m. Pacific Standard Time, 5:08 p.m. Mountain Standard Time. Farther east, as noted, the Sun has already set.
If the weather is not cooperating in your area, there's always the Internet. Slooh.com is hosting a webcast of the event.
So is the Kitt Peak National Observatory.