Summer weekends fill up fast, so you should aim to book your caterer at least three weeks before the party. Getting the date secured early saves you from scrambling for backup plans when the popular vendors are already booked.
You can manage the $850 cost by breaking it into smaller monthly payments. Using the Citi Simplicity® Credit Card, you get 0% for 18 months on purchases and balance transfers from date of account opening. This lets you spread the cost out over the 18-month window instead of paying it all at once.
Remember that the 18-month window is a deadline, not free money. If you have a balance remaining after the 18 months are up, you will start paying interest on whatever is left, which will increase the total cost of your party.