yii2 datepicker disable dates using javascript

Your date is in the wrong format. It should be specified as:

$("#w0").datepicker("setDatesDisabled", ['08/25/2017']);

Of course make sure that w0 is the correct ID for the input element... it might also be that your selector doesn't match the input.

I've verified on the demo page that entering this in the browser's console correctly disables Aug 28:

$('#sandbox-container input').datepicker("setDatesDisabled", ['08/28/2017']);