If you have an activity, which has fixed departure dates, you can use fixed dates as your date picker.
If you have an activity which has particular days that are unavailable, you can pick your “Unavailable” dates, through open dates.
Simple rule of thumb in this case -
If your activity is available for less than 183 days (half the year), use fixed dates to pick all the available days.
If your activity is available for more than 183 days (half the year), use open dates to pick all the unavailable dates.