Calendar.cs

using System;

public static class Calendar {
	public enum Month {
		JANUARY = 1, FEBRUARY, MARCH, APRIL, MAY, JUNE, JULY, AUGUST, SEPTEMBER, OCTOBER, NOVEMBER, DECEMBER
	}

	public enum EventType {
		NONE,
		CHRISTMAS,
		EASTER,
		//...
	}

	//assume events last for two weeks at a time
	public static EventType GetEvent(int day, Month month, int year, bool ignoreEaster = false) {
		//check the input
		if (!CheckValidDate(day, month, year)) {
			throw new Exception("Date is invalid: " + day + ", " + month + ", " + year);
		}

		//first, since Easter is so *special* that it feels like moving around a lot, let's calculate it separately
		//this will override any other events that might fall on these dates, so ignoreEaster is a parameter
		if (!ignoreEaster) { //if there's more moving events, you might want to add a list of strings to signal what to ignore
			//fallback on C#'s DateTime because holy crap I don't wanna touch that algorithm
			DateTime easterSunday = CalculateEasterSunday(year);
			//two weeks
			DateTime start = easterSunday.AddDays(-7);
			DateTime end = easterSunday.AddDays(6);

			DateTime now = new DateTime(year, (int)month, day);

			if (start <= now && end >= now) {
				return EventType.EASTER;
			}
		}

		//jump to the month
		switch(month) {
			case Month.JANUARY:
			case Month.FEBRUARY:
			case Month.MARCH:
			case Month.APRIL:
			case Month.MAY:
			case Month.JUNE:
			case Month.JULY:
			case Month.AUGUST:
			case Month.SEPTEMBER:
			case Month.OCTOBER:
			case Month.NOVEMBER:
				break;

			case Month.DECEMBER:
				//christmas period
				if (day >= 18 && day <= 31) {
					return EventType.CHRISTMAS;
				}
				break;
		}

		return EventType.NONE;
	}

	//helper functions
	public static bool CheckValidDate(int day, Month month, int year) {
		return day > 0 && day <= MaxDays(month, year);
	}

	public static int MaxDays(Month month, int year) {
		switch(month) {
			case Month.JANUARY:
			case Month.MARCH:
			case Month.MAY:
			case Month.JULY:
			case Month.AUGUST:
			case Month.OCTOBER:
			case Month.DECEMBER:
				return 31;

			case Month.APRIL:
			case Month.JUNE:
			case Month.SEPTEMBER:
			case Month.NOVEMBER:
				return 30;

			case Month.FEBRUARY:
				return CheckLeapYear(year) ? 29 : 28;

			default:
				throw new Exception("Unknown month");
		}
	}

	public static bool CheckLeapYear(int year) {
		//accurate up to about eight thousand years, apparently
		if (year % 400 == 0) {
			return true; //leap
		}
		if (year % 100 == 0) {
			return false;
		}
		return year % 4 == 0; //leap
	}

	public static DateTime CalculateEasterSunday(int year) {
		//praise jebus!
		int g = year % 19;
		int c = year / 100;
		int h = (c - (int)(c / 4) - (int)((8 * c + 13) / 25) + 19 * g + 15) % 30;
		int i = h - (int)(h / 28) * (1 - (int)(h / 28) * (int)(29 / (h + 1)) * (int)((21 - g) / 11));

		int day   = i - ((year + (int)(year / 4) + i + 2 - c + (int)(c / 4)) % 7) + 28;
		int month = 3;

		if (day > 31)
		{
			month++;
			day -= 31;
		}

		return new DateTime(year, month, day);
	}
}