I don't understand how one can even book for a full month O.oWell you can book a hotel for as long as you wish and pay for, and congress (and other) hotels sometimes reserve a room contingent for long-term bookings (like employees sent over for a project etc).
This is why the hotel tells you: sorry, we full for all the short-term rooms (the Radisson will be booked by attendees of other events too, not only those connected to CCH but also to Hamburg Messe. So the MARINE INTERIORS fair followed by the CRUISE DAYS are dragging many people internationally, of which quite many will already arrive during the last days of EF sadly).
Same applies to the Intercity Dammtor (which is just opposite side of the park) and the Wagner Hotel im Dammtorpalais. Just fully booked, probably because often used by businesses and not only by EF folks.