Hi there, Shannon, and welcome to planDisney!
I'm thrilled to hear you're thinking about a quick trip into the Happiest Place on Earth in a couple of weeks - my family and I were able to sneak in a one-day trip last summer at the end of another vacation and had so much fun.
If I'm understanding you correctly, it sounds like you've already purchased your
theme park tickets but were having trouble making your
theme park reservation for May 28. The reservations are subject to availability and can become unavailable if capacity is reached - it sounds like that might have been the case when you originally looked. However, people's plans change all the time and as I am answering your question and looking at the
Reservation Availability Calendar, it does look like both parks are available for reservations.
Note that this can (and likely will!) change at any point, so I would keep an eye on the reservation calendar and make your theme park reservation as soon as you're able to.
Did you also purchase the
Park Hopper option with your ticket? With that, you'll be able to hop between both parks as much as you would like after 11:00 AM! I love the flexibility this provides to be able to hit all the attractions and dining spots that you're interested in - all in one day!
I hope you have a fantastic time during your time at the Happiest Place on Earth, and at your convention after that! Thank you again for your question, Shannon.
Have a Magical Day,
Greg