Hi, Sheli! If my family and I are going to do a theme park on the day we arrive, we usually choose the one that will be open the latest. On March 4th, when you arrive, I would say you could do either the Magic Kingdom or EPCOT. My choice would be to do EPCOT, which is open until 9 o'clock, plus you get to start your vacation with a viewing of one of my favorite parts of Walt Disney World - Illuminations. This is the nightly show at EPCOT that takes place on the water, so you can stand anywhere around the World Showcase for a wonderful view.
If you want an easier afternoon and evening, so you can get some rest to be up and out even earlier the next morning, you could visit Disney's Animal Kingdom. The park closes at 5 o'clock that day, but that gives you time to enjoy at least a few of the attractions, such as Expedition Everest or the Kali River Rapids, and then it guarantees you will be back at your resort in time to relax and have dinner to rest up for a full day the next day.
You can find all of the theme park hours for the day you arrive by visiting
HERE .
I hope you all enjoy a magical vacation!