http://www.codeproject.com/KB/aspnet/ASPNETService.aspx
You can send mail automatically using the global.asax file..
Add a Global.asax file in your website and write this in Applicaation_Start event
void Application_Start(object sender, EventArgs e)
{
DateTime dt=DateTime.Now;
if(dt.Day==1)
{
//Write the code to send email
}
}
This will send mail on every month's 1st date