Legal Question in Business Law in California

Probably not. Your due date isn't the site's fault, after all. As far as I can tell, they have lived up to their end of the bargain. You are free to ask for a refund, but if the contract says they can keep your deposit and they have honored the contract then they don't have to give your money back.

This reminds me of the "Coronation Cases" which all law students study as an example of how the law of contracts deals with "frustration of purpose." You might want to put the terms in quotation marks into a Google search and read some of the results, especially the Wikipedia articles on the English case of "Krell v. Henry" and "frustration of purpose" in general.

Your situation is probably not one where you are legally entitled to recover your deposit. At best, you are released from further performance under the contract. However, getting restitution is probably not possible under the legal doctrine and cases mentioned because the events causing frustration of purpose must not be of the party's own making, causation or fault. The king's illness in Krell v. Henry was not caused by either of them, but the pregnancy was 'caused' by the parties to the dispute.

As a negotiation tactic, I would consider asking only to move the date rather than cancel. That might be a win-win solution for everyone. And most important - Double congratulations!!