Hiya, Emily!
Worst time to visit? Bite your tongue! There's no such thing as a worst time to visit. I would say that every time you go to
Walt Disney World, it will either be someone's favorite time to go or least favorite time to go.
Here's the low-down on visiting at the end of June. First, it will be hot. It is Central Florida, after all, and summer temperatures can soar into the 90s during the day. I'm pretty sure that's why they made Mickey Ice Cream Bars. You may also encounter a daily afternoon thunder storm. These storms are typically fast moving, and they are pretty easy to see as they roll in. I find I can usually do a little shopping, grab a bite to eat, or even enjoy an attraction and find the storm has passed when I come back out.
In terms of crowds, it'll be busy. Kids are done school for the summer, and it's time for family vacations. You also are planning to be there just as
Toy Story Land is opening at
Disney's Hollywood Studios on June 30, 2018. And, with the July 4th holiday just after you leave, you can expect the weekend before the holiday to be quite busy.
On the plus side, it's the best swimming weather, since it will be so warm, and you can really enjoy the water parks or even some of the attractions that cause you to get wet. There are plenty of places to try cool beverages or icy snacks, and what better time to try them out than when the temps are nice and toasty?
Have a very happy birthday celebration!
Anna