The Gregorian Calendar is the normal calendar used by most countries. (12 months and February can have either 28 or 29 days.) It was first adopted by some Catholic countries on 15 October 1582, however, many countries retained the previous Julian Calender for centuries after this date. (The UK switched calendars in 1752 but Greece held out until 1923.)
 
(The Julian Calendar year was slightly shorter than the normal earth year and key dates gradually crept into teh wrong season - the Gregorian Calendar introduced leep years to correct the problem.)
 
There are many complications when converting dates between calendars including

• Calandars can be based on the Sun (Gregorian,) the Moon (Hijri,) Sun and Moon (Chinese) or Epochs (Julian Day)
• Calendars can start their days at different times midnight (Gregorian,) sunset (Hijri), midday (Julian Day) etc.
• Calendars have years that vary in length using leep-days (Gregorian,) leep months (Hebrew) etc.
• Some calendars use lunar sightings to synchronise with lunar phases. It may be impossible to guaranntee precide date conversion until after the relevant sighting has been confirmed
• Geographic location can affect the formulas used when converting come calendars resulting. (e.g. the date-line affects the Gregorian calendar.)
• Time zones and variations in Seasonal Clock Changes can also affect results.

When calculating the dates of festivals, feasts or religious activities it is important to consider the following

• Many festivals will not occur on the same Gregorian day every year. E.g. Easter Sunday can drift bewteen March and April.
• Most (but not all) of the "moveable feast" dates can be calculated in advance. Often they are a "fixed date" within a non-Gregorian calendar or can be caluculated using a pre-determined algorhythmn.
• Festival dates can vary with country of location. E.g. Christian countries often celebrate Easter on a different date to Orthodox/Coptic countries.

DigitalDan.co.uk