getting value of client side into server side

I am writing a webpart and I have the following problem. I have a textbox declared in C# called Days. I am using the HtmlTextWriter to write this textbox to the webpart and i am using a function CheckDays in javascript to check the data on the clientside. output.AddAttribute(HtmlTextWriterAttribute.Name,"Days"); output.AddAttribute("OnChange","CheckDays(this)"); I also have a button declared called Submit. when the button is clicked I use an eventhandler called SubmitButtonClicked SubmitButton.Click += new EventHandler(SubmitButtonClicked); In this event i want to get the value of the textbox Days on the client side. Days.Text does not work, how do I get cle client side value?