Calendar with 24h

You may be able to fix it by doing the following:

1. Open the Settings on your iPhone
2. Select "Mail, Contacts, Calendars"
3. Scroll down to the Calendars section
4. Select "Time Zone Support"
5. Turn on Time Zone support

Or maybe you want to turn time zone support off? It's a little confusing. Here's what Apple says:

When Time Zone Support is on, Calendar displays event dates and times in the time zone set for your calendars. When Time Zone Support is off, Calendar displays events in the time zone of your current location.

Close Calendar app. Go to Finder. Press cmd+G to navigate to a folder. Type in ~/Library/Preferences. Locate a file called com.apple.iCal.plist. Move it to Trash.

Now try to relaunch Calendar. Hopefully you should now be able to select the calendars you want to see and have them still be selected once you quit and reopen Calendar.