Sami Fathi, MacRumors:

Despite previous attempts to put the situation at rest, some iCloud users continue to experience spam calendar invitations, causing their calendars to be filled with random events.

And:

Victims are targeted in various ways. The most common method is by receiving a normal iCloud calendar invitation through their calendar app.

Interacting with the invitation, including declining, accepting, or choosing “Maybe,” lets the spammer know that the email is valid, so it can continue to be targeted.

Other users are targeted through web pop-ups on potentially malicious or adult websites.

If you find yourself subscribed to a spam calendar event, check out the video below, which Apple Support posted a few weeks ago. Also, check out this Apple support document, which basically says the same thing as the video.