Question: I want to schedule the email with asp.net and C# code.

what is basic idea or code for schedule email?

Answer: 
Use a scheduler perhaps? [Quartz.NET](http://quartznet.sourceforge.net/) is a pretty decent one.

I assume you already know how to send a mail, so just schedule a new job, and roll with it.
Try just writing a simple console app that sends the email and using Windows Task Scheduler to run it when you need it. Sending an email is a pretty standard task, like Will said, there are plenty of similar questions pertaining to it on here already, but if you have a look at the [System.Net.Mail](http://msdn.microsoft.com/en-us/library/system.net.mail.aspx) namespace, that should get you started.