The body of the screen is a list of available occassions, covering this year, next year and the year after.
This means there is always a calendar year defined for each of the lectionary years, A,B,C.
Select an occassion by clicking on it, and then clicking the "Ok" button.
Double clicking on an occassion, does the same thing.
The "Help" button, shows this screen.
The "Cancel" button, closes the screen, ignoring any changes.
The "Ok" button, closes the screen, saving any changes.