I will refrain from making absolute claims of "easiest" or "cleanest", but  
it seems to me that you could take advantage of the Tag property of the  
controls and group them together with a group box.  Then your code would  
just enumerate all of the controls in the group box, parsing each one's  
Tag if it's checked, and or-ing that into your enumeration.

For example:

    MyEnum myenum = MyEnum.None;

    foreach (CheckBox chk in groupBox1.Controls)
    {
        if (chk.Checked)
        {
            myenum |= (MyEnum)Enum.Parse(typeof(MyEnum),  
chk.Tag.ToString());
        }
    }

If the Name or Text property of the checkboxes just happened to coincide  
with the names of the enum values, you could even just use either of those  
properties instead as appropriate.

Pete

Hi,
If you can modify the function that receives the days in a string, I
suggest use the enum as a binary flag, for example:

[Flags]
enum Days
{
    Monday = 1,
   Tuesday = 2,
   Wednesday = 4,
   Thursday = 8,
   Friday = 16,
   Saturday = 32
}

So you can manage it better using bitwise operators, to read all the
selected values:

Days d;
if (chkMonday.Checked)
    d |= Days.Monday;
if (chkTuesday.Checked)
    d |= Days.Tuesday;
...

To check whether the user choosen a day, you can simply test without
using substrings:

if (selectedDay & Days.Sunday == Days.Sunday)

Anyway, if you can't change that function, I would do it this way:

CheckBox[] chks = new CheckBox[] { chkMonday, chkTuesday,
chkWednesday, ... }
StringBuilder days = new ...();
for (int d = 0 ; d < 7 /* or chks.Length */ ; d++)
{
    if (d != 0)
         days.Append("|");
   days.Append(d);
}

Hope this helps,
Diego

If you are going to be dealing with with enum a lot then I would have an
utility class that takes boolean values, which you can pass the checkbox
Checked property.

public static DaysOfWeek Build(
bool sunday,
bool monday,
bool tuesday,
bool wednesday,
bool thursday,
bool friday,
bool saturday)
{
DaysOfWeek daysOfWeek = new DaysOfWeek();
if(sunday) daysOfWeek = daysOfWeek | DaysOfWeek.Sunday;
if(monday) daysOfWeek = daysOfWeek | DaysOfWeek.Monday;
.......
return DaysOfWeek;
}

PS

It would be easier for you if the enumeration declaration is tagged with the
Flags attribute.
Then the enum would be or-able.

You can create your text boxes in any of the ways suggested by other posters
and then pass a single value containing the sum of the selected days.