var startdate = "@Date(2014;12;1)";
var enddate = "@Date(2014;12;31)";
var weekends = "1:7"; //assuming your weekend is Saturday and Sunday
var holidays = "@Date(2014;12;24):@Date(2014;12;25)"; // assuming your holiday is 24th and 25th December
var formula = "@BusinessDays(" + startdate + ";" + enddate + ";" + weekends + ";" + holidays + ")";
return session.evaluate(formula);