A private pizza class with five friends is a great way to spend a weekday evening. Booking during the week usually gets you a better rate and more flexibility with the chef's schedule.
You want to lock in your date at least a month out to coordinate everyone. Once the date is set, you can manage the upfront cost without draining your checking account.
The 18-month window is a deadline, not a suggestion. If you still have a balance once those 18 months are up, the card will start charging interest on whatever is left over.