AI overview
February 30 is a date that cannot exist in the Gregorian calendar because February never has more than 29 days, even in leap years. It's sometimes used as a humorous or symbolic reference to something impossible or nonexistent.
AI-generated from the Wikipedia summary — may contain errors.
Article
February 31 on a tombstone Several non-standard dates are used in calendars for various purposes: some are expressly fictional, some are intended to produce a rhetorical effect (such as sarcasm), and others attempt to address a particular mathematical, scientific or accounting requirement or discrepancy within the calendar system.
Historical