Begin with the person
Choose a small number of things the person would enjoy, rather than a long list of things an occasion is supposed to include. A meal, a shared activity or a thoughtful message can provide enough structure for the day.
Work back from the chosen date
Use the add-days tool with a negative number to find a planning date before the occasion. Make a note of any task that depends on someone else, such as confirming a venue or inviting a guest. Keep those commitments in your own calendar.
Keep the plan flexible
A celebration need not happen on the exact anniversary. If another day makes it easier to gather, keep the anniversary and the event date as two separate entries. A clear distinction avoids confusion when you send invitations.