$("#selector").datepicker({ beforeShowDay: highlightDays });
...
var dates = [new Date("1/1/2011"), new Date("1/2/2011")];
function highlightDays(date) {
for (var i = 0; i < dates.length; i++) {
if (date - dates[i] == 0) {
return [true,'', 'TOOLTIP'];
}
}
return [false];
}