The RetainSelectedDay property allows you to control how the selected
cell behaves when the calendar month or year changes. If this property is
set to true, the selected cell will retain the same day as the currently
selected day, as the month or year is changed. If the day does not exist
in the next month or year, the selected cell will be the next highest day.
This property also works if the selected cell is currently on an "extra"
day.
