Submitting value from a hidden form field

I have a form inserting into an access database, 1 of the fields is hidden and required.
When I click submit the insert fails, if I change the visible value to "true" the insert succeeds.
I know this is probably really simple but I can't figure it out.If you set a controls visibility to false then it doesn't appear in the rendered HTML, therefore your hidden field won't be there at all (check the HTML source). In order to make a control still contain a value but be readonly try using the enabled property and set it to fale instead eg

<asp:TextBox id="myTextBox" enabled="false" runat="server"/>

That way the value is retained but it can't be changed.

Other option is to use a 'traditional' <input type="hidden"> field.
Thanks but the value has to be editable. The hidden field is actually receiving a date that is selected from a calendar on the page using this:

void Selection_Change(Object sender, EventArgs e)
 {
 countDate.Text = calendar.SelectedDate.ToShortDateString();
 }

Where countDate is the hidden field.

If there is an easier way to do this please let me know.
As was explained,

you could use:

<input type="hidden"runat="server" ID="countDate"
Then just the code would be:

countDate.Value = calendar.SelectedDate.ToShortDateString();

Thanks, that worked.

I now have another problem, when changing the date on the calendar it appears to be trying to submit the form, causing error messages as some other fields are also required.