First of all, the book is called The Long Walk not the Walk. Second, the very first line of chapter five says, "At twenty minutes of ten on that endless May first..." This is at the end of the first night.
I'd like to point out that you DID read the German version, and while I haven't read the Bachman Books in a while, it occured to me that someone may have messed up the translation, and it may not be the fault of Stephen King.
In the french version of the book, the date given is also the 31st April, so it's probably no translation mistake, but indeed a slipup by S. King. Or... maybe the crazy fascist commander had the calendar changed because he or his mother was born in April or something like that (like Roman emperor August actually did with his own month).
In the Bachman series it says the same thing. Also, on page 291 it has the mistake of "April 31st blackout date". Perhaps it was a date made up in the future?