best calendar app for iOS is free for 2 days...

Calendars 5 by Readdle is hands down the best iOS calendar app I have used.... and I have used a LOT of calendar apps for iPhone and iPad. You can go here to learn more about this app and others Readdle makes, but even better news is (if you are reading this in a timely manner) you may be able to get it for free. (Sunday May 18th and Monday May 19th)

If you want to see some of the other options and learn more about why iMore picked this as the best iOS calendar app, check out their review here