How to convert date to word format?

You'll need to look at each date part and use a function to get the written equivalent. I've included a class below that converts integers to written text, and extended it to support DateTime conversion as well:

public static class WrittenNumerics
{
    static readonly string[] ones = new string[] { "", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine" };
    static readonly string[] teens = new string[] { "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen" };
    static readonly string[] tens = new string[] { "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety" };
    static readonly string[] thousandsGroups = { "", " Thousand", " Million", " Billion" };

    private static string FriendlyInteger(int n, string leftDigits, int thousands)
    {
        if (n == 0)
            return leftDigits;

        string friendlyInt = leftDigits;
        if (friendlyInt.Length > 0)
            friendlyInt += " ";

        if (n < 10)
            friendlyInt += ones[n];
        else if (n < 20)
            friendlyInt += teens[n - 10];
        else if (n < 100)
            friendlyInt += FriendlyInteger(n % 10, tens[n / 10 - 2], 0);
        else if (n < 1000)
            friendlyInt += FriendlyInteger(n % 100, (ones[n / 100] + " Hundred"), 0);
        else
            friendlyInt += FriendlyInteger(n % 1000, FriendlyInteger(n / 1000, "", thousands + 1), 0);

        return friendlyInt + thousandsGroups[thousands];
    }

    public static string DateToWritten(DateTime date)
    {
        return string.Format("{0} {1} {2}", IntegerToWritten(date.Day), date.ToString("MMMM"), IntegerToWritten(date.Year));
    }

    public static string IntegerToWritten(int n)
    {
        if (n == 0)
            return "Zero";
        else if (n < 0)
            return "Negative " + IntegerToWritten(-n);

        return FriendlyInteger(n, "", 0);
    }
}

_{Disclaimer: Basic functionality courtesy of @Wedge}

Using this class, just call the DateToWritten method:

var output = WrittenNumerics.DateToWritten(DateTime.Today);

The output of the above is: Twelve May Two Thousand Twelve

This isn’t what you want, but the closest I can suggest using built-in functionality is ToLongDateString, which gives you the name of the month and is obviously culture-sensitive.

string str = bdate.ToLongDateString();
// Assuming en-US culture, this would give: "Saturday, May 12, 2012"