Ограничение выбора в календарной форме

$('#datepicker').datepicker({ 
  beforeShowDay: function(date) { 
    return [date.getDay() === 1] 
  } 
});

<link href="https://code.jquery.com/ui/jquery-ui-git.css" rel="stylesheet" /> 
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> 
<script src="https://code.jquery.com/ui/1.12.1/jquery-ui.min.js"></script> 
 
<input type="text" id="datepicker">