Note- 1st click on the calendar button then click on the date which you want to select.
Here in the below example, it will always select the current date , you can pass any date which you want to select as per your requirement. 

import java.util.Calendar;
import java.util.GregorianCalendar;
import java.util.concurrent.TimeUnit;

import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.firefox.FirefoxDriver;

    public class TodayDate {
    public static void main(String[] args) {
        int day, month, year;
          GregorianCalendar date = new GregorianCalendar();
          day = date.get(Calendar.DAY_OF_MONTH);
          month = date.get(Calendar.MONTH)+1;
          year = date.get(Calendar.YEAR);
          String today = “a_”+year+”_”+month+”_”+day;
          System.out.println(today);
    WebDriver driver = new FirefoxDriver();
    driver.manage().timeouts().implicitlyWait(5, TimeUnit.SECONDS);
    driver.get(“http://www.yatra.com/”);
    driver.findElement(By.id(“BE_flight_depart_date”)).click();
    driver.findElement(By.xpath(“//a[@id='”+today+”‘]”)).click();
    
    }

    }

 

Source: http://selenium-makeiteasy.blogspot.com