To receive updated data for synchronized calendars and calendar subscriptions regularly please check that your iOS System Settings are correct.
- Select device Settings
- Select Passwords & Accounts (Accounts & Passwords in iOS11, Calendar in iOS10 or Mail, Contacts, Calendars in iOS9 and below)
- Select Fetch New Data (in the Accounts section)
- Set it to Push for regular accounts (then they will be refreshed automatically) and
set all Calendar Subscriptions to Fetch Every 15 Minutes.