This is the date that the calender is reckoned as having started on. In some cases (French Revolution, and Islam) it did start on that date. In other cases the exact start date cannot be identified. In the case of the Gregorian the chosen start date was the date that Jesus Christ was believed to have been born. We now know that he must have been born in 4 BC or earlier. The day and month are not known. For the actual date that the Gregorian calender started in each country see the Gregorian page.
This is the average number of days in each year taken over the full cycle of leap days / months, even though the calender may not have been in operation long enough to complete the first cycle.
This is the number of periods of a length close to the lunar cycle into which the year is divided. The name 'month' may not be used by the calendar in question.
This is a short explanation of the way in which the year length is corrected to match the tropical year. This is usually done by inserting an extra day into the calendar at regular intervals. Years when this happens are known as 'Leap Years'.