Datepicker: alert when choosing a specific weekday

Hi, i need to have an alert when a user selects a sunday. I tried a lot without results. We already running a script on the datepicker form to hide tuesday and wednesday. All the stuff i tried had issues with that script.

How can i do this? Any advice?

What have you tried so far? Can you share any of your work?

and more

it conflicted with the already existing javascript on that datepicker: screenshot:

Also, whatever i try: i get the error: jQuery(…).datepicker is not a function
