JavaFX with Scene Builder: creating a datePicker where you can pick exactly 2 dates.

Controller.java

package sample;

import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.DateCell;
import javafx.scene.control.DatePicker;
import javafx.scene.input.MouseEvent;

import javafx.util.Callback;
import java.net.URL;
import java.time.LocalDate;
import java.util.ResourceBundle;

public class Controller implements Initializable {

    ObservableList<LocalDate> selectedDates = FXCollections.observableArrayList();

    @FXML
    private DatePicker datePicker;

    @FXML
    void getDate(MouseEvent event) {
        for (LocalDate date : selectedDates) {
            System.out.println(date);
        }
    }

    @Override
    public void initialize(URL url, ResourceBundle resourceBundle) {
        datePicker.setOnAction(event -> {
            if (selectedDates.size() > 1){
                selectedDates.clear();
            }
            selectedDates.add(datePicker.getValue());}
            );

        datePicker.setDayCellFactory(new Callback<DatePicker, DateCell>() {
            @Override
            public DateCell call(DatePicker param) {
                return new DateCell() {
                    @Override
                    public void updateItem(LocalDate item, boolean empty) {
                        super.updateItem(item, empty);
                        boolean alreadySelected = selectedDates.contains(item);
                        setDisable(alreadySelected);
                        setStyle(alreadySelected ? "-fx-background-color: #C06C84;" : "");
                    }
                };
            }
        });
    }
}

Main.Java

package sample;

import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;

public class Main extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception{
        Parent root = FXMLLoader.load(getClass().getResource("sample.fxml"));
        primaryStage.setTitle("Hello World");
        primaryStage.setScene(new Scene(root));
        primaryStage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }
}

sample.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.control.Button?>
<?import javafx.scene.control.DatePicker?>
<?import javafx.scene.layout.AnchorPane?>


<AnchorPane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/15.0.1" xmlns:fx="http://javafx.com/fxml/1" fx:controller="sample.Controller">
   <children>
      <Button layoutX="88.0" layoutY="168.0" mnemonicParsing="false" onMouseClicked="#getDate" prefHeight="65.0" prefWidth="122.0" text="getDates" />
      <DatePicker fx:id="datePicker" layoutX="306.0" layoutY="188.0" />
   </children>
</AnchorPane>

You can need to reopen the datePicker to pick a second date and then when you pick the 3rd date it reset to just having 1 day picked.