To receive updated data for synchronized calendars and calendar subscriptions regularly please see if your iOS System Settings are correct.
- Select device Settings
- Select Calendar (or Mail, Contacts, Calendars in iOS9 and below)
- In the (or below for iOS9 and below) Accounts section, select Fetch New Data
- Set it to Push for regular accounts (then they will be refreshed automatically) and
set all Calendar Subscriptions to Fetch Every 15 Minutes