Getting day suffix when using DateTime.ToString()

As a reference I always use/refer to [SteveX String Formatting] 1 and there doesn't appear to be any "th" in any of the available variables but you could easily build a string with

string.Format("{0:dddd dd}{1} {0:MMMM yyyy}", DateTime.Now, (?));

You would then have to supply a "st" for 1, "nd" for 2, "rd" for 3, and "th" for all others and could be in-lined with a "? :" statement.

var now = DateTime.Now;
(now.Day % 10 == 1 && now.Day % 100 != 11) ? "st"
: (now.Day % 10 == 2 && now.Day % 100 != 12) ? "nd"
: (now.Day % 10 == 3 && now.Day % 100 != 13) ? "rd"
: "th"

Using a couple of extension methods:

namespace System
{
    public static class IntegerExtensions
    {
        public static string ToOccurrenceSuffix(this int integer)
        {
            switch (integer % 100)
            {
                case 11:
                case 12:
                case 13:
                    return "th";
            }
            switch (integer % 10)
            {
                case 1:
                    return "st";
                case 2:
                    return "nd";
                case 3:
                    return "rd";
                default:
                    return "th";
            }
        }
    }   

    public static class DateTimeExtensions
    {
        public static string ToString(this DateTime dateTime, string format, bool useExtendedSpecifiers)
        {
            return useExtendedSpecifiers 
                ? dateTime.ToString(format)
                    .Replace("nn", dateTime.Day.ToOccurrenceSuffix().ToLower())
                    .Replace("NN", dateTime.Day.ToOccurrenceSuffix().ToUpper())
                : dateTime.ToString(format);
        } 
    }
}

Usage:

return DateTime.Now.ToString("dddd, dnn MMMM yyyy", useExtendedSpecifiers: true);
// Friday, 7th March 2014

Note: The integer extension method can be used for any number, not just 1 to 31. e.g.

return 332211.ToOccurrenceSuffix();
// th

Another option using switch:

string GetDaySuffix(int day)
{
    switch (day)
    {
        case 1:
        case 21:
        case 31:
            return "st";
        case 2:
        case 22:
            return "nd";
        case 3:
        case 23:
            return "rd";
        default:
            return "th";
    }
}