A single parameter specifies the year (1 - 9999) to be displayed; note the year must be fully specified: ``cal 89 '' will not display a calendar for 1989. Two parameters denote the month (1 - 12) and year. If no parameters are specified, the current month's calendar is displayed.
A year starts on Jan 1.
The Gregorian Reformation is assumed to have occurred in 1752 on the 3rd of September. By this time, most countries had recognized the reformation (although a few did not recognize it until the early 1900's.) Ten days following that date were eliminated by the reformation, so the calendar for that month is a bit unusual.