We have done the Chef's Table on two occasions, and were not able to select the day when booking. We were informed of the day once onboard. They create the schedule and determine the number of days it will be offered based on the amount of demand there is on that sailing.
On our most recent sailing, the date conflicted with other plans that we had already made, and they were able to switch to date to a time convenient for us.
As mentioned in a previous reply, confirm that it will be available for your sailing, as the Chef's Table had been temporarily suspended.