Several potential arguments:
1. Liquidated damages vs. penalty: Courts distinguish between legitimate liquidated damages (a reasonable pre-estimate of actual harm) and unenforceable penalties. If the venue can rebook your date (especially with 6+ months notice), keeping $20K may be deemed a penalty. The venue's actual damages are the difference between your contract price and what they can get from a replacement booking — possibly zero.
2. Unconscionability: Some courts have found event venue cancellation clauses unconscionable, especially when the consumer had no meaningful bargaining power and the clause is buried in a lengthy contract.
3. Impossibility/impracticability: A serious family illness might support a defense of impossibility, though this is harder to argue for personal events vs. commercial contracts.
Start by writing a formal letter requesting a partial refund, citing your willingness to allow them to rebook the date. If they refuse, small claims court (up to $10K in most states) is a cost-effective option.