4 of the contract. This made it impracticable for Laird to perform but the contract makes no mention of Laird's performance. The unavailability of Laird was a risk the Seller assumed. Promise and condition. The fact that Seller was in breach does not necessarily mean Buyer does not have to pay.
Buyer is excused only if the breach is material.
The contract makes the completion date a promise. Buyer will claim that she told Seller that the date was important. However, I don't see that in the written contract. Seller can still atone with damages like the cost of moving the wedding reception to a hotel. Buyer must still pay but can deduct damages.