Just try using xpath
String month="Sept"; String date="28"; "//td[text()='"+month+"']/../..//a[text()='"+date+"']"
He will choose September 28 
Indicate the month and date based on your requirements.
The logic below is for navigating between months.
driver.findElement(By.cssSelector("td.next")).click(); driver.findElement(By.cssSelector("td.previous")).click();

I hope this works (I have not tried it on my machine)
EDIT-I
I tried the logic in my machine and it works fine.
driver=new FirefoxDriver(); driver.get("http://www.redbus.in"); //selecting date of journey driver.findElement(By.id("calendar")).click(); driver.findElement(By.xpath("//td[text()='Sept']/../..//a[text()='27']")).click(); //selecting return jouney driver.findElement(By.id("calendar1")).click(); driver.findElement(By.xpath("//td[text()='Oct']/../..//a[text()='3']")).click();
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