Cheltenham always used to be the second Tuesday of March - no matter how early. I can recall it falling on 9th March before now.
These days it seems to be whichever week St Patrick's Day falls in, so it has begun to fall a week later. I suppose it is so that our Irish visitors can use their national holiday as part of their week off work (also lets the local pubs and clubs benefit from the celebrations), but it can muck up the schedule for the Aintree-bound horses.
Edward, however, has said that Cheltenham and Aintree had recently agreed to ensure 3 weeks between their major meetings.