C#+WPFで作る日めくり式カレンダー(仮) ref: http://qiita.com/yukkuribemani/items/6f9fe61f7f10a20c0aa3

MainWindow.xaml.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
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 WpfCallender
{
    /// <summary>
    /// MainWindow.xaml の相互作用ロジック
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

            DateTime date = DateTime.Today;
            HolidayJudge(date);
            Today.Text = date.ToString("yyyy/MM/dd");

            BeforeDay.Click += (s, e) =>
            {
                date = date.AddDays(-1);
                Today.Text = date.ToString("yyyy/MM/dd");
                HolidayJudge(date);
            };

            NextDay.Click += (s, e) =>
            {
                date = date.AddDays(1);
                Today.Text = date.ToString("yyyy/MM/dd");
                HolidayJudge(date);
            };
        }

        public void HolidayJudge(DateTime today)
        {
            if (today.DayOfWeek == DayOfWeek.Saturday || today.DayOfWeek == DayOfWeek.Sunday)
            {
                MessageBox.Show("今日は休みです!! やったね!!!");
            }
        }
    }
}