This method will return a correctly inclined czech month based on the passed month number and case (C# 8 and C#9 ver.)

CzechMonths.cs

static readonly string[] DeclensedCzechMonthNamesBase = { "ledn", "únor", "březn", "dubn", "květn", "červn", "červenc",
        "srpn", "září", "říjn", "listopad", "prosinc"};

static readonly string[] FullCzechMonthNames = { "leden", "únor", "březen", "duben", "květen", "červen", "červenec",
        "srpen", "září", "říjen", "listopad", "prosinec" };



// C#8
public static string GetCzechMonthName(byte month, byte @case = 1)
{
    if (@case < 1 || @case > 7)
      throw new ArgumentException($"case must be within the range 1-7 (was {@case})");
        
    if (month < 1 || month > 12)
      throw new ArgumentException($"Month must be within the range 1-12 (was {month})");

    string @base = DeclensedCzechMonthNamesBase[month - 1];

    if (month == 9)
      return @base + (@case == 7 ? "m" : "");
    
    // C# 8 switch expression - change to if-else or switch statement if your c# version is < 8.
    return @case switch
    {
      var x when x == 3 || x == 6 => @base + (month == 7 || month == 12 ? "i" : "u"),
      2 => @base + (month == 9 || month == 12 ? "e" : (month == 11 ? "u" : "a")),
      7 => @base + (month == 9 ? "m" : "em"),
      _ => FullCzechMonthNames[month - 1]
    };
}

// C#9
public static string GetCzechMonthName(byte month, byte @case = 1)
{
    if (@case is < 1 or > 7)
        throw new ArgumentException($"case must be within the range 1-7 (was {@case})");

    if (month is < 1 or > 12)
        throw new ArgumentException($"Month must be within the range 1-12 (was {month})");

    string @base = DeclensedCzechMonthNamesBase[month - 1];

    if (month is 9)
        return @base + (@case is 7 ? "m" : "");

    // C# 8 switch expression and C#9 pattern matching.
    return @case switch
    {
        byte x when x is 3 or 6 => @base + (month is 7 or 12 ? "i" : "u"),
        2 => @base + (month is 9 or 12 ? "e" : (month is 11 ? "u" : "a")),
        7 => @base + (month is 9 ? "m" : "em"),
        _ => FullCzechMonthNames[month - 1]
        };
    }