When choosing a date for a hotel stay, the system will show at various times during the buying process the date that you chose in the calendar.
Dates that are not available are always blacked out in the calendar. If by any chance you try reserving for a date that is not available, the system will not allow the purchase to proceed. Only available dates (marked in blue) will be able to be chosen.
If you chose the wrong date for your reservation, verify if the promotion you acquired has the option to change the date. If the option is available, you may see this article for more information.