My iPhone is reminding as the data is leaked

Open your calendar app and tap Calendar at the bottom, if there is a calendar you do NOT know, tap it and delete it. You can also go to Settings -> Calendar -> Accounts. If there is an account you do not know, delete it immediately.

On some websites there are pop ups where there are invitations to participate in a kind of "SPAM calendar" you have clicked add without thinking about it. I have tried it myself and you just delete it again.