Validating form data that it is a weekday I'm trying to validate a custom form field from an Ubercart module (in the checkout process). The field would specify a delivery date. The delivery date must be a business day.

How does one determine if it is a business day or not? 

Are there given functions within Drupal that provide which day of the week when a response is given? Or would native php functions be the route to go with?
date('w'[, $timestamp]);


gives you the days of the week as an integer from 0 (Sun) to 6 (Sat).

So you could check to see whether the chosen delivery date is 1 - 5.
I am not aware of any existing Drupal function to do this, but it's a fairly simple function to write oneself.