<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525"
        DataContext="{Binding RelativeSource={RelativeSource Self}}">
    <Grid>
        <Calendar SelectedDate="{Binding SelectedDate}" SelectedDatesChanged="TrySelectDate"/>
    </Grid>
</Window>

using System;
using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Windows;
using System.Windows.Controls;

namespace WpfApplication1
{
    public partial class MainWindow : Window, INotifyPropertyChanged
    {
        private DateTime? _selectedDate;
        public DateTime? SelectedDate
        {
            get { return _selectedDate; }
            set
            {
                _selectedDate = value;
                NotifyPropertyChanged();
            }
        }

        public MainWindow()
        {
            InitializeComponent();
        }

        public void TrySelectDate(object sender, SelectionChangedEventArgs args)
        {
            var date = args.AddedItems[0] as DateTime?;
            if (date == null)
            {
                return;
            }

            // For testing... if user saved.
            var saved = false;

            if (!saved)
            {
                if (args.RemovedItems.Count > 0)
                {
                    var prevDate = args.RemovedItems[0] as DateTime?;
                    if (prevDate != null)
                    {
                        SelectedDate = prevDate;
                    }
                }
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
        protected void NotifyPropertyChanged([CallerMemberName] string name = "")
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(name));
            }
        }
    }
}