You made the mistake of writing you automation without taking your domain into consideration. Let look at your selector
The first part is absolutely fine, you go straight and root yourself in the interesting sub component. On the second value, you define that september (data-month=9) ...
Try like below and confirm:
WebDriverWait wait = new WebDriverWait(driver,30);
// Click on the "Date of journey"
// try to click on the date since past dates will be disabled.