JQuery Datepicker - only select mondays and thursdays -

I have the following code which only allows users to select Monday from jquery datepicker.

I want

  Show before: Function (date) {return [date.getDay () == 1, "" <">  < / Pre> 

You can add one or ( = ), in this way: Show before

 : function (date) {var day = date.getDay (); return [day == 1 || day == 4, ""];}  

It has been shown only once that the cost per day is one, it is not that it is an expensive operation, but there is no reason.