How to have an elert that not allow people enter anything in the input box?

If people try enter any chars in the input box, we would like to have an alert message saying "Please calendar to select the date." Please help me how to do that? Thanks so much for your help.

You don't want them to be able to enter anything in? Why not just make it readonly...?

thanks for asking. I have been thinking that too, but I just want them to know that a calendar is available there for them to use.
By the way, a current date displays in the input box, if they want to change the date....we just want them to use a calendar...therefore, we would like to have an alert message to let them know. Is it possible to do it?

<input type="text" name="whatever" onkeypress="alert('foo'); return false;">

THANKS Brandoe85. You're a man :thumbsup: