If you attempt to visit the Taj Mahal on a day trip from Delhi you are setting yourself up for disappointment in my opinion.

It is a 4 to 5 hour drive by car to reach Agra. So even if you leave Delhi at 0600 you won't arrive to Agra until mid-morning at best. It will be miserably hot and crowded at the site; you will come away wondering what all the hype was about. Then you face another 4 to 5 hour drive back.

Instead spend some extra money and stay the night in Agra (if your trip budget permits it, check into the Oberoi Amarvillas!). Get up and get to the Taj at sunrise before the crowds and the heat of the day. You'll have a much better experience.