Function to return a date of the Catholic calendar

[Seddryck]

The function calendar-catholic(event) is accepting an argument defining the event of the Christian calendar. It returns the date of the event for the year specified in the argument. The name of the event is not case-sensitive. If the event is not defined (out of the list defined below), the function returns null.

2023 | calendar-catholic("Christmas")
=> 2023-12-25

2023 | calendar-catholic("Easter Sunday")
=> 2023-04-09

2023 | calendar-catholic("Corpus Christi")
=> 2023-06-08

2023 | calendar-catholic("First Sunday of Advent")
=> 2023-12-03

2022 | calendar-catholic("First Sunday of Advent")
=> 2022-11-27

• Epiphany: 6th of January
• Candlemas: 2nd of February
• The Annunciation: 25th of March
• Shrove Tuesday: 46 days before Easter Sunday (is a Tuesday)
• Ash Wednesday: 45 days before Easter Sunday (is a Wednesday)
• Palm Sunday: 7 days before Easter Sunday (is a Sunday)
• Maundy Thursday: 3 days before Easter Sunday (is a Thursday)
• Good Friday: 2 days before Easter Sunday (is a Friday)
• Easter Sunday: See calculation below (is a Sunday)
• Ascension Day: 39 days after Easter Sunday (is a Thursday)
• Pentecost (alias Whit Sunday): 49 days after Easter Sunday (is a Sunday)
• Whit Monday: 50 days after Easter Sunday (is a Monday)
• Trinity Sunday: 56 days after Easter Sunday (is a Sunday)
• Corpus Christi: 61 days after Easter Sunday (is a Thursday)
• The Assumption: 15th of August
• Immaculate Conception: 8th of September
• All Saints’ Day: 1st of November
• First Sunday of Advent: 4 Sundays before Christmas (is a Sunday)
• Christmas: 25th of December

The function to calculate Easter is defined here and documented at https://www.tondering.dk/claus/cal/easter.php

private static DateTime Easter(int year) {
    int a = year%19;
    int b = year/100;
    int c = (b - (b/4) - ((8*b + 13)/25) + (19*a) + 15)%30;
    int d = c - (c/28)*(1 - (c/28)*(29/(c + 1))*((21 - a)/11));
    int e = d - ((year + (year/4) + d + 2 - b + (b/4))%7);
    int month = 3 + ((e + 40)/44);
    int day = e + 28 - (31*(month/4));
    return new DateTime(year, month , day);
}