using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace Berber
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            DataContext = new Müşteri();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            var müşteri = DataContext as Müşteri;
            DateTime gün = DateTime.Today.AddDays(müşteri.Gün);
            if (gün.DayOfWeek==DayOfWeek.Wednesday)
            {
               gün=gün.AddDays(1);
            }
            müşteri.Mesaj = $"Sayın {müşteri.AdSoyad} Berberimizden Almış Olduğunuz {müşteri.İşlem} randevusu {string.Format("{0:dd/MM/yyyy dddd}", gün)} günüdür.";
        }
    }
}

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;

namespace Berber
{
    public class Müşteri : INotifyPropertyChanged
    {
        private string adSoyad;
        private string işlem;
        private string mesaj;
        private int gün;
        private IEnumerable<int> günler=Enumerable.Range(1,8);

        public string AdSoyad
        {
            get { return adSoyad; }

            set
            {
                if (adSoyad != value)
                {
                    adSoyad = value;
                    OnPropertyChanged(nameof(AdSoyad));
                }
            }
        }
        public string İşlem
        {
            get { return işlem; }
            set
            {
                if (işlem != value)
                {
                    işlem = value;
                    OnPropertyChanged(nameof(İşlem));
                }
            }
        }
        public string Mesaj
        {
            get { return mesaj; }
            set
            {
                if (mesaj != value)
                {
                    mesaj = value;
                    OnPropertyChanged(nameof(Mesaj));
                }
            }
        }
        public int Gün
        {
            get { return gün; }
            set
            {
                if (gün != value)
                {
                    gün = value;
                    OnPropertyChanged(nameof(Gün));
                }
            }
        }

        public IEnumerable<int> Günler
        {
            get { return günler; }

            set
            {
                if (günler != value)
                {
                    günler = value;
                    OnPropertyChanged(nameof(Günler));
                }
            }
        }


        protected virtual void OnPropertyChanged(string propertyName) => PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));

        public event PropertyChangedEventHandler PropertyChanged;
    }
}

<Window x:Class="Berber.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:local="clr-namespace:Berber"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:system="clr-namespace:System;assembly=mscorlib"
    Title="MainWindow"
    Width="400" Height="244"
    mc:Ignorable="d">
    <Grid d:DataContext="{d:DesignInstance Type=local:Müşteri}">
        <Grid.Resources>
            <x:Array x:Key="İşlemler" Type="{x:Type system:String}">
                <system:String>Sakal Traşı</system:String>
                <system:String>Saç Traşı</system:String>
                <system:String>Etek Traşı</system:String>
            </x:Array>
        </Grid.Resources>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition />
            <ColumnDefinition />
        </Grid.ColumnDefinitions>
        <TextBlock Text="ADI SOYADI" TextWrapping="Wrap" />
        <TextBox Text="{Binding AdSoyad, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Grid.Row="1" TextWrapping="Wrap" />
        <TextBlock Text="İŞLEM" Grid.Row="2" TextWrapping="Wrap" />
        <ComboBox Grid.Row="3" ItemsSource="{Binding Mode=OneWay, Source={StaticResource İşlemler}}" SelectedItem="{Binding İşlem}" />
        <TextBlock Text="Randevu Aralığı" Grid.Row="4" TextWrapping="Wrap" />
        <ListBox Grid.Row="5"
            ItemsSource="{Binding Günler}"
            SelectedItem="{Binding Gün}"
            SelectionMode="Single">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding Mode=OneWay, StringFormat=\{0\} GÜN}" />
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
        <Button Content="Mesaj Oluştur"
            Grid.Row="1" Grid.Column="1"
            Click="Button_Click" />
        <TextBlock Text="Gönderilecek Mesaj"
            Grid.Row="2" Grid.Column="1"
            TextWrapping="Wrap" />
        <TextBox Text="{Binding Mesaj}"
            Grid.Row="3" Grid.Column="1" Grid.RowSpan="3"
            TextWrapping="Wrap" />

    </Grid>
</Window>