Hi Traci,
Good news, for a typical one-day, one-park ticket, there is no expiration! From the Walt Disney World web site
Frequently Asked Questions section:
"Walt Disney World Resort continues to honor all unexpired theme park tickets with remaining admission days."
Tickets purchased from someone other than Disney may have an expiration date so check your tickets carefully. I know tickets we won in a school raffle once had an expiration date, but thankfully the date was several years away so we had plenty of time to use the tickets.
Fortunately, you should be able to purchase one day tickets and use them when the weather is fantastic!
Thanks for stopping by the Disney Parks Moms Panel with your question. Please be sure to let me know if I can help you with any other questions!
Wishing you a magical day,
Amy H