Give this a go:
string format = "{0} / {1} / {2} {3}";
string date = string.Format(format,mon.ToString(),da.ToString(),yer.ToString();
Console.WriteLine(date);
In fact, there's probably a way to format it automatically without even doing it yourself.
Check out http://msdn.microsoft.com/en-us/library/8kb3ddd4.aspx