ReadLine returns a string, so you can't assign it to an integer value. You need to look at it first, make sure it is a number and then convert it to one.
Try this:
int n;
Console.WriteLine("enter your choice ");
string s = Console.ReadLine();
if (!int.TryParse(s, out n))
{
Console.WriteLine("\"{0}\" is not a number", s);
}
else
{
switch (n)
{
case 1: if (n == 1)
Console.WriteLine("1:Monday");
break;
case 2: if (n == 2)
Console.WriteLine("2:Tuesday");
break;
case 3: if (n == 3)
Console.WriteLine("3:Wednesday");
break;
case 4: if (n == 4)
Console.WriteLine("4:Thursday");
break;
case 5: if (n == 5)
Console.WriteLine("5:friday");
break;
case 6: if (n == 6)
Console.WriteLine("6:saturday");
break;
case 7: if (n == 7)
Console.WriteLine("7:sunday");
break;
default: Console.WriteLine("wrong choice");
break;
}
}