GetCatholicEaster.cs

/// <summary>
/// Get Catholic easter for requested year
/// </summary>
/// <param name="year">Year of easter</param>
/// <returns>DateTime of Catholic Easter</returns>
public static DateTime GetCatholicEaster(int year)
{
    var month = 3;

    var a = year % 19 + 1;
    var b = year / 100 + 1;
    var c = (3 * b) / 4 - 12;
    var d = (8 * b + 5) / 25 - 5;
    var e = (5 * year) / 4 - c - 10;

    var f = (11 * a + 20 + d - c) % 30;
    if (f == 24)
        f++;
    if ((f == 25) && (a > 11))
        f++;

    var g = 44 - f;
    if (g < 21)
        g = g + 30;

    var day = (g + 7) - ((e + g) % 7);
    if (day > 31)
    {
        day = day - 31;
        month = 4;
    }
    return new DateTime(year, month, day);
}

GetOrthodoxEaster.cs

/// <summary>
/// Get Orthodox easter for requested year
/// </summary>
/// <param name="year">Year of easter</param>
/// <returns>DateTime of Orthodox Easter</returns>
public static DateTime GetOrthodoxEaster(int year)
{
    var a = year % 19;
    var b = year % 7;
    var c = year % 4;

    var d = (19 * a + 16) % 30;
    var e = (2 * c + 4 * b + 6 * d) % 7;
    var f = (19 * a + 16) % 30;

    var key = f + e + 3;
    var month = (key > 30) ? 5 : 4;
    var day = (key > 30) ? key - 30 : key;

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