Make a hash sum of the file.
Publish the hash of the document somewhere. Like on your Mastodon timeline or on Reddit, or on HN, or as an ad in a national printed newspaper.
Make sure the text is pretty long though. If you write something super short someone could guess the hash for it.
Write a short comment along with the hash. Something like:
“My prediction will be revealed on Dec 31 2025. Sha256 a9774a1a6ebf564cc408cfd86b5f2c06c13d830e143989714d958d34f325db13.”
When time comes, publish the document.