When something is annual, it happens once a year. An annual holiday party should be a time for fun, but it also can be a sad yearly reminder of the passage of time.
An annual event happens every year: "I always forget to take sunglasses to my annual eye exam and am left wearing those horrible disposable shades." Annual earnings are made over the course of a year: "His annual income is on a steady decline." And an annual plant lives for only a year: "My neighbor Shirley always plants the most garishly bright annuals instead of more subdued perennials." Whether it's eye exams, income, or plants, you can expect to see something annual again in about 365 days.