I'm puzzled by the replies.
You don't have to warn the property in advance and for any stay over any weekend at any rate (it doesn't however make sense to book a 241 rate as the second night is already free) - you simply present the certificate at check in and the second weekend night of your stay is not charged for. The certificate will work for any length of stay in any country including Saturday arrival to Monday departure in all countries where Sunday night is a weekend night. So for example if you stay in Cologne from Saturday to Wednesday your Sunday night is your free night. You cannot choose it - it is a certificate condition.
Some elements MAY not be included as the cert entitles you to accomodation only but in practise properties exercise thta right differently and I cannot see any particular advantage in having the debate by e-mail. Some feel you might compromise your place in the upgrade queue by decalring the cert before you arrive.
i tried to do the saturday-monday in germany last time and was denied by the hotel and told only friday-sunday .
i tried thurday to saturday too and was denied by the IC athens for that too.
maybe my expirience has been wrong?
dp