How to convert a char array to a string array

char[] daysCodeArray = days.ToCharArray();
string[] daysArray = daysCodeArray.Select(el =>
{
    switch (el)
    {
        case 'M':
            return "Monday";

        case 'T':
            return "Tuesday";

        case 'W':
            return "Wednesday";

        case 'R':
            return "Thursday";

        case 'F':
            return "Friday";

        case 'S':
            return "Saturday";

        case 'U':
            return "Sunday";
     }
     throw new ArgumentException("Invalid day code");
}).ToArray();

You can change the lambda into a separate method if you want.

Just using char.ToString() would work:

var daysArray = days.ToCharArray().Select( c => c.ToString()).ToArray();

Alternatively, and a better solution in my mind why don't you use the string directly with a dictionary for the mapping:

var daysArray = days.Select( c => dayMapping[c]).ToArray();

with dayMapping just a Dictionary<char, string> that maps to the full day name:

Dictionary<char, string> dayMapping = new Dictionary<char,string>()
{
    {  'M', "Monday" },
    {  'T', "Tuesday" }
    //and so on
}