Hi, Denise! If you are celebrating something, then Walt Disney World is celebrating something with you! By all means, if you can't be at Walt Disney World for an actual birthday, celebrate when you can be there!
For quite a few of our family occasions, like my daughter's high school graduation and a Brownie bridging (we celebrate everything with a trip to Walt Disney World!), we've had to postpone a trip due to schedules, school, or other events. I have never felt badly about noting on a dining reservation that we were celebrating or ordering something special from the Disney Florist (www.disneyflorist.com).
Besides, how could a company who popularized the UN-birthday have a problem with you celebrating when it's not actually your birthday?
Have a wonderful visit to Walt Disney World, and enjoy a very happy celebration!