April 23rd is the day of the prerelease, they will be in stores on April 25th.
Well I'm certain that there are prereleases this weekend, and generally pokemon sets are released during the week, so I'm fairly sure it's on Monday. However, I'm sure you're right Donavannj that a lot of places will put them out whenever they get them just to get the edge. And actually LAF, the prereleases were uncharacteristically late for this set, generally they occur two weeks before the set's release, not the week of. I'm not sure why they did it so late this time around.
Well, some areas apparently had pre-releases last weekend, based on what a recent poster in the booster pack thread posted.