Random sale ad in Calendar
Seeing the event is unnerving but, thankfully, it is unlikely to be a virus or current problem with your MacBook.
Instead it is possible this event came from an unwanted e-mail; Mail.app on macOS and on iOS has the ability to add events discovered in your e-mails.
You can disable this feature using the steps from How to disable suggested events in Calendar on iOS and Mac:

How to disable Mail event suggestions on OS X
Do the following on your Mac with OS X 10.11 El Capitan or later:

Launch Mail.
Choose Preferences from the Mail menu.
Click the General tab.
Select Never in the drop-down menu labeled “Add invitations to Calendar”.