About the format it says "Other lines should begin with a month and day. They may be entered in almost any format, either numeric or as character strings." So

About the format it says "Other lines should begin with a month and day. They may be entered in almost any format, either numeric or as character strings." So "month/day" isn't a requirement. And there's no reason the dates can't be placed in their true order--calendar.ubuntu is ordered by version number and calendar.debian has multiple groups of dates ordered by year.