You can achive this by following way:
Day render event:
void cal_DayRender(object sender, DayRenderEventArgs e)
{
CheckBox chk = new CheckBox();
chk.ID = "cal" + e.Day.Date.ToShortDateString();
if (!String.IsNullOrEmpty(Request.Form[chk.ID]))
chk.Checked = true;
e.Cell.Controls.Add(chk);
}
Button click event
protected void btnSubmit_Click(object sender, EventArgs e)
{
string calKeys = "";
string[] selectedDates=null;
for (int i = 0; i < Request.Form.Keys.Count; i++)
{
if(Request.Form.Keys[i].ToString().Contains("cal"))
calKeys += Request.Form.Keys[i].ToString() + ",";
}
if (calKeys != string.Empty)
{
calKeys = calKeys.Trim(',');
selectedDates = calKeys.Split(',');
for (int i = 0; i < selectedDates.Length; i++)
{
selectedDates[i] = selectedDates[i].Substring(3);
}
}
lblMessage.Text = string.Join("," , selectedDates);
}