It is possible to visit both Disneyland Park and Walt Disney Studios Park in a single day, but it requires a very disciplined itinerary and usually the purchase of a Disney Premier Access pass. Disneyland Paris is unique because its two parks are located right next to each other, making "park hopping" extremely easy. To succeed in one day, you should arrive at "rope drop" (opening time) and start at Walt Disney Studios Park to finish high-demand rides like Crush’s Coaster and Ratatouille before the crowds peak. By midday, you can walk over to the main Disneyland Park, which is significantly larger and more beautiful. Because the main park contains the majority of the iconic attractions like Big Thunder Mountain and Pirates of the Caribbean, you will likely spend 70% of your day there. However, be prepared to skip smaller flat rides and character meet-and-greets if you want to experience all the major "E-ticket" attractions. If you are a first-time visitor who wants to soak in the atmosphere and see the parades and fireworks without feeling rushed, two days is much more realistic.