The Main Tab is where all the basic data for the school year is set up. You define the First and Last Days of school, choose the iCal Calendar destination, and enter a Location if you wish. You can also change the Default Year at any time on this Tab.
After you have finished with all of your Date information, you return to this Tab to click the Done - Send to iCal button.
The initial dates that appear in the date pickers are, of course, just suggestions. All the changes you make to the dates are written to the default database, and will remain until you change them again. Changing the Default Year ONLY alters the year; the month and day you define is stable.
This is where the main dates for the school year are set up: First day, Last day, and the typical vacation periods. The initial date values are only suggestions, of course, that should be pretty close to the correct dates for your school district, thus minimizing the amount of editing necessary to enter the actual scheduled dates.