Determine the type of calendar the original date was used in. This could include a historic date from the Julian calendar, or it could be part of the Hebrew or Islamic calendars. There have been numerous calendars used throughout history, so it is important to determine the one used for the original date.
Assess how the original calendar calculated the number of days in a cycle. Is it a lunar- or solar-based calendar? Are the days and weeks grouped according to religious events? Many societies have also created calendars based on historic events, such as the French Republican calendar created during the Revolution in 1793.
Consider the length of a year in the original calendar to that of the Gregorian. An average year in the Gregorian calendar is 365.2425 days.
Depending on the length of a cycle or year in the original calendar, you may be able to create a comparison chart to find the comparable day in the Gregorian calendar. This can be extremely complicated, however, especially for religious or historical calendars.
Calendar converters online assist in converting one day to another in many different types of calendars.