What is date format for 07th November 2021?

The tricky part is the "th", becasue it has to mutate to "st"/"nd"/"rd" for the some days of a month.

So these days require special-casing of this ordinal suffix, e.g:

string ordinalSuffix;
if (date.Day == 1 || date.Day == 21 || date.Day == 31)
    ordinalSuffix = "st";
else if (date.Day == 2 || date.Day == 22)
    ordinalSuffix = "nd";
else if (date.Day == 3 || date.Day == 23)
    ordinalSuffix = "rd";
else
    ordinalSuffix= "th";

string formatted = date.ToString($"dd'{ordinalSuffix}' MMMM yyyy");

You surely are aware that this works for English only. To support other languages, you might consider using a library like Humanizer.

Try to use this function here:

        public static string ToStringWithSuffix(this DateTime dt, string format) {   
        // The format parameter MUST contain [$suffix] within it, which will be replaced.   
        int day = dt.Day; string suffix = "";   
        // Exception for the 11th, 12th, & 13th   
        // (which would otherwise end up as 11st, 12nd, 13rd)   
        if (day % 100 >= 11 && day % 100 <= 13) {   
            suffix = "th";   
        }else{
            switch (day % 10) {   
                case 1: 
                    suffix = "st";   
                    break;   
                case 2:
                    suffix = "nd";   
                    break;   
                case 3:   
                    suffix = "rd";   
                    break;  
                default:   
                    suffix = "th";   
                    break;   
            }
        }
        // Convert the date to the format required, then add the suffix.   
    return dt.ToString(format).replace("[$suffix]",suffix);
}

You can call it like so:

DateTime(2021, 2, 21);
Console.WriteLine(dt.ToStringWithSuffix("dd'[$suffix]' MMMM yyyy"));

You can also check the documentation on Custom date and time format strings if you need other formats.

Source: https://gist.github.com/woodss/006f28f01dcf371c2bd1