You agreed to pay for a party on April 2nd. Having agreed to pay for the party, you are required to pay for it unless the birthday location was the one that caused you to miss it. However, based on what you write, it was your own mistake; and when person A agrees to pay person B for something, and person A doesn't get that thing due to person A's mistake, person A still must pay for it. You can't get out of a contractual obligation (i.e. your agreement) due to your own actions or error.