Quote from page 15 of the paperback edition of Notes From A Small Island - "No-one in the whole of Dover would have any particular reason to remember 21st March 1973 except for me and a handful of children who were born that day and possibly one old guy with a dog who had encountered a young fellow with underpants on his head."
Here, I think he's talking about the date on which he found somewhere to stay. He'd spent the previous night on a park bench, so probably arrived on the 20th. It's not entirely clear.
Cheers