About the same time every year my right ankle and foot becomes swollen. What causes that?

Shoes/activity. May be associated with the seasons activity or shoe style. You will need to be a detective. Less likely is arthritis that only flares up with the weather.
Annual ankle. Many people during the year perform activities specific to that season. Those people who go skiing in winter or ice-skating or do specific activities in the summer such as mountain biking or other sports activities specific to that season will sometimes experience overuse injury. If already predisposed to ankle or foot abnormalities these activities can then cause injury or swelling or pain.