customized WordPress plugin code

Hi there there,I’ve a WordPress web site that wants a easy appointment reserving part. Every little thing is working fantastic however I need to exclude Sundays from the calendar. I discovered this code however I do not know the place it ought to go. I attempted inserting it within the features. Php however I get an error message. The place ought to it go to be able to make it work appropriately? Or you probably have a greater suggestion, I would recognize it.

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