It's possible you're thinking of:
* National Punctuation Day: Celebrated annually on September 24th, which might include discussions about homonyms as part of the focus on proper grammar and usage.
* National Grammar Day: Celebrated annually on March 4th, where homonyms are often featured in activities and discussions about language.
However, there's no formal event dedicated specifically to homonyms.
If you're interested in learning more about homonyms, there are many resources available online and in libraries.