[SOLVED]
Sorry, didn't check the config file carefully... there's already such
option:
# if $options{calendar_select_order} is blank (totally commented out),
calendars will
# be listed on the dropdown menu in the order they were created.
# You can change this by setting $options{calendar_select_order} to
"alpha" (sort alphabetically)
# or by explicitly ordering the calendars by id(you may have to look
at the calendar data to find the id).
$options{calendar_select_order} = "alpha";
#$options{calendar_select_order} = "0,3,4,1,6,7,8";