Received notification from calender

 If running iOS 13 or earlier versions, check: Settings - Passwords & Accounts - Accounts - Any rogue entries here? If so, delete the rogue account. 

If running iOS 14, check: Settings - Calendar - Accounts - Any rogue entries here? If so, delete the rogue account. 

Are you seeing something as shown in pic below?

Then probably you have accepted an offer or subscribed for a spam Calendar accidentally. Watch the GIF below to know how to unsubscribe from these alerts. Look for an unknown Calendar on the device under subscribed calendars.

iOS / iPadOS devices cannot be hacked or infected with Virus / Malware / Spyware, unless you have intentionally downloaded spurious softwares or unauthorised apps directly from internet and installed on your device or/and have Jail Broken.

A scam.

Abdullah_Noraiz wrote:

But how this is possible that notification continously appearing from alerts of calendar...?

Because you accepted an offer from a web site to “Send Notifications”.

You are welcome.