It is unclear as to why there is such a specific date especially since there has not been any announcement made by HTC regarding a launch event, so we are a bit skeptical of those claims at the moment. That being said, there are several other smartwatches due for a refresh so maybe we could learn more about those then as well.
Now a possibility is that towards the end of the month, Google will be hosting their I/O event and while it was not specified, the HTC smartwatch could be Android Wear powered, meaning that Google/HTC will announce the device then, and the smartwatch will be made available around June for purchase, but that is just one possibility.
We do know that the device is real. Earlier this year HTC had confirmed that they were working on a smartwatch that would turn the industry on its head, whatever that means, so until we actually see the device for ourselves, take this with a grain of salt for now.