Note that the exact date in May has not yet been announced, but there is a hint: in all the videos and screenshots of displays the date and time is shown as "Mon 25 10:10".

Now when Ive was still at Apple, the first screenshots of iPhone showed a time of 9:42 because that was the time they expected when the device was first shown. And that time was placed in all the official PR images well in advance.

Extracting from that, a Monday the 25th could be the time we'll first see the full car. Going through all 25ths of each month this year, May is indeed the only month where it falls on a Monday, so it's probably the 25th of May.