What is this Russian calendar reminder that popped up?

It appears to be this: https://www.androidpolice.com/2019/06/22/google-calendar-phising-scam/

Scammers are exploiting [Google] Calendar['s] default setting, which automatically adds invites to your agenda, even if you haven't accepted them. Even worse, it'll create an automatic reminder to notify you a few minutes before the fake event takes place. [...] The invitation would typically contain a link to collect sensitive data such as your credit card or bank account number. Since the notification comes from an app people tend to trust, they'd pay less attention to its authenticity, which is precisely what attackers want.

The article also mentions how to fix this google calendar bug (ahem, I mean 'feature' of course):

[to deactivate] automatic event creation[,] head over to Calendar's settings using a computer, navigate to Event Settings -> Automatically add invitations -> No, only show invitations to which I've responded. You can also prevent Calendar from showing declined events in your agenda by going to View Options and deselecting Show declined events.