My current planner has smaller rings so I don't carry a full 12 months. So, how do I handle making appointments that go beyond the three months that are in my planner? I use the future planning insert that you can download below. I keep it directly behind the last month in my planner and if I need to add an appointment and I don't have the month available, I will write it on that insert.
When it is time to change out my monthly pages, I use that sheet to fill in the dates I need to remember. Simple yet effective!