Well, the AddDate method adds up the digits contained in whatever string is passed to it until it eventually arrives at a single digit.
So, if Saturday produces a total of 22 and you want to reduce this to the single digit 4, then you need to pass total.ToString() i.e. "22" to the method.
Your code should therefore be:
string text1 = comboBox2.SelectedItem.ToString().ToLower();
int total1 = 0;
for (int i = 0; i < text1.lenght; i++)
total1 += dict[text1[i]];
int number = AddDate(total1.ToString());
textBox3.Text = number.ToString();