The first step I would recommend would be chadscott's suggestion. We have some guides on how to clear your cache and cookies, depending on the browser
If you have done this, and are still seeing the problem, I would suggest switching browsers. Canvas Support generally recommends Chrome or Firefox. While Safari is also supported, that browser can run into issues with different aspects in Canvas. Please, also make sure your browser is up to date.
Secondly, I would also suggest Oliver69's idea for turning off calendars that you no longer need to view. If you are still seeing problems with the Calendar after these steps, I would recommend contacting Support.