Hack message

iOS / iPadOS devices cannot be hacked or infected with Virus / Malware / Spyware if it is updated to the latest iOS/iPadOS unless you have intentionally downloaded spurious software or unauthorized apps directly from the internet and installed on your device or/and have Jail Broken. 

It (Hacking) also depends on how careful are you in sharing sensitive and valuable information pertaining to your iPhone such as Passcode, Password etc with your friends and family members.

Be careful when sharing the device's sensitive and valuable information with friends and family members.

Thumb Rules:

1. Enable  Guided Access before handing over an iPhone to kids
2. Don't share Apple IDs
3. Don't Jail Break
4. Don't share sensitive information pertaining to your device and payment methods
5. Beware of Phishing

1. Open the Calendar app.
2. At the bottom of the screen, tap Calendars.
3. Look for a calendar that you don't recognize. Tap the More Info button  next to that calendar, then scroll down and tap Delete Calendar.

If this doesn't fix the issue, delete the calendar subscription in Settings:

1. Open the Settings app.
2. Tap Calendar > Accounts. Or if you use iOS 13, tap Passwords & Accounts > Accounts instead.
3. Tap Subscribed Calendars.
4. Look for a calendar that you don't recognize. Tap it, then tap Delete Account.