Maggie,
My recollection is that Radisson usually finalizes the excursion lists sometime the day prior to excursions. I believe they will announce the cut-off time for making reservations, and I would think that would be the cut-off time for cancellations also. As Donna said, I believe that time usually will be about 24 hours before the excursion, but may be more or less depending on a particular excursion.
