You have to do it on the client side.
One way to get the dropdownlist on the client side:
var ddl_location = document.getElementById("<%=ddlLocation.ClientID%>");
Then populate the dropdownlist with array data:
if ddl_location {
for (var i=0; i < month.length;++i){
addOption(ddl_location, some_text_variable, some_value_variable);
}
}
addOption = function(selectbox, text, value) {
var optn = document.createElement("OPTION");
optn.text = text;
optn.value = value;
selectbox.options.add(optn);
}