Any time between September through March is a good time to visit. Winters here are cool and sometimes quite cold in December and January, so make sure you have some layers packed if you visit at that time.

If you try to pack all of Agra's sights into a day that starts in Delhi, you'll have a long, tiring day; starting fresh in the morning from Agra will make for a full, but fairly easy day. Consider staying a second night at your hotel and dividing your excursions between two days to eliminate that constant hectic feeling that Agra's crowds, congested traffic, and sights often impart.